NCERT Solutions for Class 11 Maths Chapter 14 Mathematical Reasoning

*According to the latest update on the CBSE Syllabus 2023-24, this chapter has been removed.

The NCERT Solutions for Class 11 Maths Chapter 14 Mathematical Reasoning prepared by subject-matter experts at BYJU’S can be accessed on this page, and students can prepare well for the Class 11 annual exam. A majority of the problems in the textbook exercises are included in the PDF given below, which enables the students to learn the concepts easily and revise for the examination. NCERT Solutions are simple and easy to comprehend. These solutions enable Class 11 students to understand the concept of Mathematical Reasoning accurately.

NCERT Solutions for Class 11 Maths Chapter 14 Mathematical Reasoning

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Get detailed NCERT Solutions for Class 11 Maths Chapter 14 for all exercises below:

Exercise 14.1 Solutions: 2 Questions (Short Answers)

Exercise 14.2 Solutions : 3 Questions (Short Answers)

Exercise 14.3 Solutions: 4 Questions (Short Answers)

Exercise 14.4 Solutions: 4 Questions (Short Answers)

Exercise 14.5 Solutions: 5 Questions (Short Answers)

Miscellaneous Exercise Solutions: 7 Questions (Short Answers)

Access Answers to NCERT Solutions Maths Chapter 14 Mathematical Reasoning

Exercise 14.1 Page No. 324

1. Which of the following sentences are statements? Give reasons for your answer.

(i) There are 35 days in a month.

(ii) Mathematics is difficult.

(iii) The sum of 5 and 7 is greater than 10.

(iv) The square of a number is an even number.

(v) The sides of a quadrilateral have equal lengths.

(vi) Answer this question.

(vii) The product of (–1) and 8 is 8.

(viii) The sum of all interior angles of a triangle is 180°.

(ix) Today is a windy day.

(x) All real numbers are complex numbers.

Solution:

(i) The maximum number of days in a month is 31, so this sentence is incorrect. Therefore, it is a statement.

(ii) This sentence is subjective. For some people, Mathematics can be easy, and for others, it can be difficult. Therefore, it is not a statement.

(iii) The sum of 5 and 7 is 12, and it is greater than 10. Therefore, this sentence is always correct. Hence, it is a statement.

(iv) This sentence can be sometimes correct and sometimes incorrect. For example, the square of 2 is an even number, but the square of 3 is an odd number. Hence, it is not a statement.

(v) This sentence can be sometimes correct and sometimes incorrect. For example, squares and rhombi have sides of equal lengths, whereas trapezia and rectangles have sides of unequal lengths. Therefore it is not a statement.

(vi) It is an order. Hence, it is not a statement.

(vii) The given sentence is incorrect because the product of (-1) and 8 is -8. Hence, it is a statement.

(viii) The given sentence is correct, and therefore, it is a statement.

(ix) The given sentence is not a statement because the day that is being referred to is not evident from the sentence.

(x) The given sentence is always correct because all real numbers can be written as a × 1 + 0 × i. Hence, it is a statement.

2. Give three examples of sentences which are not statements. Give reasons for the answers.

Solution:

The three examples of sentences which are not statements are given below:

(i) He is a doctor.

In the given sentence, it is not evident to whom ‘he’ is referred to. Hence, it is not a statement.

(ii) Geometry is difficult.

For some people, geometry can be easy, and for others, it can be difficult. Hence, this is not a statement.

(iii) Where is she going?

In this question, it is not evident to whom ‘she’ is referred to. Hence, it is not a statement.

Exercise 14.2 Page No. 329

1. Write the negation of the following statements:

(i) Chennai is the capital of Tamil Nadu.

(ii) is not a complex number.

(iii) All triangles are not equilateral triangles.

(iv) The number 2 is greater than 7.

(v) Every natural number is an integer.

Solution:

(i) Chennai is not the capital of Tamil Nadu.

(ii) is a complex number.

(iii) All triangles are equilateral triangles.

(iv) The number 2 is not greater than 7.

(v) Every natural number is not an integer.

2. Are the following pairs of statements negations of each other?

(i) The number x is not a rational number.

The number x is not an irrational number.

(ii) The number x is a rational number.

The number x is an irrational number.

Solution:

(i) The negation of the first statement is ‘the number x is a rational number’.

This is the same as the second statement because if a number is not an irrational number, then the number is a rational number.

Hence, the given statements are negations of each other.

(ii) The negation of the first statement is ‘the number x is not a rational number. This means that the number x is an irrational number which is the same as the second statement.

Hence, the given statements are negations of each other.

3. Find the component statements of the following compound statements and check whether they are true or false.

(i) Number 3 is prime, or it is odd.

(ii) All integers are positive or negative.

(iii) 100 is divisible by 3, 11 and 5.

Solution:

(i) The component statements are

(a) Number 3 is prime

(b) Number 3 is odd

Here, both statements are true.

(ii) The component statements are as follows:

(a) All integers are positive

(b) All integers are negative

Here, both statements are false.

(iii) The component statements are as follows:

(a) 100 is divisible by 3

(b) 100 is divisible by 11

(c) 100 is divisible by 5

Here, the statements (a) and (b) are false, and (c) is true.

Exercise 14.3 Page No. 334

1. For each of the following compound statements, first identify the connecting words and then break them into component statements.

(i) All rational numbers are real and all real numbers are not complex.

(ii) Square of an integer is positive or negative.

(iii) The sand heats up quickly in the Sun and does not cool down fast at night.

(iv) x = 2 and x = 3 are the roots of the equation 3x² – x – 10 = 0.

Solution:

(i) In this sentence, ‘and’ is the connecting word

The component statements are as follows:

(a) All rational numbers are real

(b) All real numbers are not complex

(ii) In this sentence, ‘or’ is the connecting word

The component statements are as follows:

(a) Square of an integer is positive

(b) Square of an integer is negative

(iii) In this sentence, ‘and’ is the connecting word

The component statements are as follows:

(a) The sand heats up quickly in the Sun

(b) The sand does not cool down fast at night

(iv) In this sentence, ‘and’ is the connecting word

The component statements are as follows:

(a) x = 2 is the root of the equation 3x² – x – 10 = 0

(b) x = 3 is the root of the equation 3x² – x – 10 = 0

2. Identify the quantifier in the following statements and write the negation of the statements.

(i) There exists a number which is equal to its square.

(ii) For every real number x, x is less than x + 1.

(iii) There exists a capital for every state in India.

Solution:

(i) Here, the quantifier is ‘there exists’.

The negation of this statement is as follows:

There does not exist a number which is equal to its square.

(ii) Here, the quantifier is ‘for every’.

The negation of this statement is as follows:

There exist a real number x, such that x is not less than x + 1.

(iii) Here, the quantifier is ‘there exists’.

The negation of this statement is as follows:

In India, there exists a state which does not have a capital.

3. Check whether the following pair of statements is a negation of each other. Give reasons for the answer.

(i) x + y = y + x is true for every real number x and y.

(ii) There exists real numbers x and y for which x + y = y + x.

Solution:

The negation of statement (i) is as given below:

There exist real numbers x and y for which x + y ≠ y + x

Now, this statement is not the same as statement (ii).

Hence, the given statements are not a negation of each other.

4. State whether the “Or” used in the following statements is “exclusive “or” inclusive. Give reasons for your answer.

(i) Sun rises or Moon sets.

(ii) To apply for a driving licence, you should have a ration card or a passport.

(iii) All integers are positive or negative.

Solution:

(i) It is not possible for the Sun to rise and the Moon to set together. Hence, the ‘or’ in the given statement is exclusive.

(ii) Since a person can have both a ration card and a passport to apply for a driving license. Hence, the ‘or’ in the given statement is inclusive.

(iii) Since all integers cannot be both positive and negative. Hence, the ‘or’ in the given statement is exclusive.

Exercise 14.4 Page No. 338

1. Rewrite the following statement with “if-then” in five different ways conveying the same meaning.

If a natural number is odd, then its square is also odd.

Solution:

The five different ways of the given statement can be written as follows:

(i) A natural number is odd, indicating that its square is odd.

(ii) A natural number is odd only if its square is odd.

(iii) For a natural number to be odd, it is necessary that its square is odd.

(iv) It is sufficient that the number is odd for the square of a natural number to be odd.

(v) If the square of a natural number is not odd, then the natural number is not odd.

2. Write the contrapositive and converse of the following statements.

(i) If x is a prime number, then x is odd.

(ii) If the two lines are parallel, then they do not intersect in the same plane.

(iii) Something that is cold implies that it has a low temperature.

(iv) You cannot comprehend geometry if you do not know how to reason deductively.

(v) x is an even number implies that x is divisible by 4

Solution:

(i) The contrapositive of the given statement is as follows:

If a number x is not odd, then x is not a prime number.

The converse of the given statement is as follows:

If a number x is odd, then it is a prime number.

(ii) The contrapositive of the given statement is as follows:

If two lines intersect in the same plane, then the two lines are not parallel.

The converse of the given statement is as follows:

If two lines do not intersect in the same plane, then they are parallel.

(iii) The contrapositive of the given statement is as follows:

If something does not have a low temperature, then it is not cold.

The converse of the given statement is as follows:

If something is at a low temperature, then it is cold.

(iv) The contrapositive of the given statement is as follows:

If you know how to reason deductively, then you can comprehend geometry.

The converse of the given statement is as follows:

If you do not know how to reason deductively, then you cannot comprehend geometry.

(v) The given statement can be written as ‘if x is an even number, then x is divisible by 4’.

The contrapositive of the given statement is as follows:

If x is not divisible by 4, then x is not an even number.

The converse of the given statement is as follows:

If x is divisible by 4, then x is an even number.

3. Write each of the following statements in the form “if-then”.

(i) You get a job implies that your credentials are good.

(ii) The Banana trees will bloom if it stays warm for a month.

(iii) A quadrilateral is a parallelogram if its diagonals bisect each other.

(iv) To get A⁺ in the class, it is necessary that you do the exercises in the book.

Solution:

(i) If you get a job, then your credentials are good.

(ii) If the Banana trees stay warm for a month, then the trees will bloom.

(iii) If the diagonals of a quadrilateral bisect each other, then it is a parallelogram.

(iv) If you want to score an A⁺ in the class, then you do all the exercises in the book.

4. Given statements in (a) and (b). Identify the statements given below as contrapositive or converse of each other.

(a) If you live in Delhi, then you have winter clothes.

(i) If you do not have winter clothes, then you do not live in Delhi.

(ii) If you have winter clothes, then you live in Delhi.

(b) If a quadrilateral is a parallelogram, then its diagonals bisect each other.

(i) If the diagonals of a quadrilateral do not bisect each other, then the quadrilateral is not a parallelogram.

(ii) If the diagonals of a quadrilateral bisect each other, then it is a parallelogram.

Solution:

(a) If you live in Delhi, then you have winter clothes.

(i) If you do not have winter clothes, then you do not live in Delhi [Contrapositive of statement (a)].

(ii) If you have winter clothes, then you live in Delhi [Converse of statement (a)]

(b) If a quadrilateral is a parallelogram, then its diagonals bisect each other.

(i) If the diagonals of a quadrilateral do not bisect each other, then the quadrilateral is not a parallelogram [Contrapositive of statement (b)].

(ii) If the diagonals of a quadrilateral bisect each other, then it is a parallelogram [Converse of statement (b)].

Exercise 14.5 Page No. 342

1. Show that the statement

p: “If x is a real number such that x³ + 4x = 0, then x is 0” is true by

(i) direct method

(ii) method of contradiction

(iii) method of contrapositive

Solution:

Let p: ‘If x is a real number such that x³ + 4x = 0, then x is 0’

q: x is a real number such that x³ + 4x = 0

r: x is 0

(i) We assume that q is true to show that statement p is true and then show that r is true.

Therefore, let statement q be true

Hence, x³ + 4x = 0

x (x² + 4) = 0

x = 0 or x² + 4 = 0

Since x is real, it is 0.

So, statement r is true.

Hence, the given statement is true.

(ii) By contradiction, to show statement p to be true, we assume that p is not true.

Let x be a real number such that x³ + 4x = 0 and let x ≠ 0

Hence, x³ + 4x = 0

x (x² + 4) = 0

x = 0 or x² + 4 = 0

x = 0 or x² = -4

However, x is real. Hence, x = 0, which is a contradiction since we have assumed that x ≠ 0.

Therefore, the given statement p is true.

(iii) By the contrapositive method, to prove statement p to be true, we assume that r is false and prove that q must be false.

∼r: x ≠ 0

Clearly, it can be seen that

(x² + 4) will always be positive

x ≠ 0 implies that the product of any positive real number with x is not zero.

Now, consider the product of x with (x² + 4)

∴ x (x² + 4) ≠ 0

x³ + 4x ≠ 0

This shows that statement q is not true.

Hence, it proved that

∼r ⇒ ∼q

Hence, the given statement p is true.

2. Show that the statement “For any real numbers a and b, a² = b² implies that a = b” is not true by giving a counter-example.

Solution:

The given statement can be written in the form of ‘if then’ is given below:

If a and b are real numbers such that a² = b², then a = b

Let p: a and b are real numbers such that a² = b²

q: a = b

The given statement has to be proved false. To show this, two real numbers, a and b, with a² = b², are required such that a ≠ b.

Let us consider a = 1 and b = – 1

a² = (1)²

= 1 and

b² = (-1)²

= 1

Hence, a² = b²

However, a ≠ b

Therefore, it can be concluded that the given statement is false.

3. Show that the following statement is true by the method of contrapositive.

p: If x is an integer and x² is even, then x is also even.

Solution:

Let p: If x is an integer and x² is even, then x is also even

Let q: x be an integer and x² be even

r: x is even

By the contrapositive method, to prove that p is true, we assume that r is false and prove that q is also false

Let x is not even

To prove that q is false, it has to be proved that x is not an integer or x² is not even.

x is not even indicates that x² is also not even.

Hence, statement q is false.

Therefore, the given statement p is true.

4. By giving a counter example, show that the following statements are not true.

(i) p: If all the angles of a triangle are equal, then the triangle is an obtuse angled triangle.

(ii) q: The equation x² – 1 = 0 does not have a root lying between 0 and 2.

Solution:

(i) Let q: All the angles of a triangle are equal

r: The triangle is an obtuse angled triangle

The given statement p has to be proved false.

To show this, the required angles of a triangle should not be an obtuse angle.

We know that sum of all the angles of a triangle is 180⁰. Therefore, if all three angles are equal, then each angle measures 60⁰, which is not obtuse.

In an equilateral triangle, all angles are equal. However, the triangle is not an obtuse-angled triangle.

Hence, it can be concluded that the given statement p is false.

(ii) The given statement is

q: The equation x² – 1 = 0 does not have a root lying between 0 and 2.

This statement has to be proven false

To show this, let us consider

x² – 1 = 0

x² = 1

x = ± 1

One root of the equation x² – 1 = 0, i.e. the root x = 1, lies between 0 and 2

Therefore, the given statement is false.

5. Which of the following statements are true and which are false? In each case, give a valid reason for saying so.

(i) p: Each radius of a circle is a chord of the circle.

(ii) q: The centre of a circle bisects each chord of the circle.

(iii) r: Circle is a particular case of an ellipse.

(iv) s: If x and y are integers such that x > y, then –x < –y.

(v) t: √11 is a rational number.

Solution:

(i) The given statement p is false.

As per the definition of a chord, it should intersect the circle at two distinct points.

(ii) The given statement q is false.

The centre will not bisect that chord which is not the diameter of the circle.

In other words, the centre of a circle only bisects the diameter, which is the chord of the circle.

(iii) The equation of an ellipse is,

If we put a = b = 1, then, we get

x² + y² = 1, which is an equation of a circle

Hence, a circle is a particular case of an ellipse.

Therefore, statement r is true.

(iv) x > y

By the rule of inequality

-x < – y

Hence, the given statement s is true.

(v) 11 is a prime number

We know that the square root of any prime number is an irrational number.

Therefore √11 is an irrational number.

Hence, the given statement t is false.

Miscellaneous Exercise Page No. 345

1. Write the negation of the following statements:

(i) p: For every positive real number x, the number x – 1 is also positive.

(ii) q: All cats scratch.

(iii) r: For every real number x, either x > 1 or x < 1.

(iv) s: There exists a number x such that 0 < x < 1.

Solution:

(i) The negation of statement p is given below:

There exists a positive real number x, such that x – 1 is not positive.

(ii) The negation of statement q is given below:

There exists a cat which does not scratch.

(iii) The negation of statement r is given below:

There exists a real number x, such that neither x > 1 nor x < 1.

(iv) The negation of statement s is given below:

There does not exist a number x, such that 0 < x < 1.

2. State the converse and contrapositive of each of the following statements:

(i) p: A positive integer is prime only if it has no divisors other than 1 and itself.

(ii) q: I go to a beach whenever it is a sunny day.

(iii) r: If it is hot outside, then you feel thirsty.

Solution:

(i) Statement p can be written in the form ‘if then’ as follows:

If a positive integer is prime, then it has no divisors other than 1 and itself.

The converse of the statement is given below:

If a positive integer has no divisors other than 1 and itself, then it is prime.

The contrapositive of the statement is given below:

If a positive integer has divisors other than 1 and itself, then it is not prime.

(ii) The given statement can be written as follows:

If it is a sunny day, then I go to a beach.

The converse of the statement is given below:

If I go to a beach, then it is a sunny day.

The contrapositive of the statement is given below:

If I do not go to a beach, then it is not a sunny day.

(iii) The converse of statement r is given below

If you feel thirsty, then it is hot outside.

The contrapositive of statement r is given below:

If you do not feel thirsty, then it is not hot outside.

3. Write each of the statements in the form “if p, then q”.

(i) p: It is necessary to have a password to log on to the server.

(ii) q: There is a traffic jam whenever it rains.

(iii) r: You can access the website only if you pay a subscription fee.

Solution:

(i) The statement p in the form ‘if then’ is as follows:

If you log on to the server, then you have a password.

(ii) The statement q in the form ‘if then’ is as follows:

If it rains, then there is a traffic jam.

(iii) The statement r in the form ‘if then’ is as follows:

If you can access the website, then you pay a subscription fee.

4. Rewrite each of the following statements in the form “p if and only if q”.

(i) p: If you watch television, then your mind is free, and if your mind is free, then you watch television.

(ii) q: For you to get an A grade, it is necessary and sufficient that you do all the homework regularly.

(iii) r: If a quadrilateral is equiangular, then it is a rectangle, and if a quadrilateral is a rectangle, then it is equiangular.

Solution:

(i) You watch television if and only if your mind is free.

(ii) You get an A grade if and only if you do all the homework regularly.

(iii) A quadrilateral is equiangular if only if it is a rectangle.

5. Given below are two statements:

p: 25 is a multiple of 5.

q: 25 is a multiple of 8.

Write the compound statements connecting these two statements with “And” and “Or”. In both cases, check the validity of the compound statement.

Solution:

The compound statement with ‘And’ is as follows

25 is a multiple of 5 and 8

This is a false statement because 25 is not a multiple of 8.

The compound statement with ‘Or’ is as follows:

25 is a multiple of 5 or 8.

This is a true statement because 25 is not a multiple of 8, but it is a multiple of 5.

6. Check the validity of the statements given below by the method given against them.

(i) p: The sum of an irrational number and a rational number is irrational (by contradiction method).

(ii) q: If n is a real number with n > 3, then n² > 9 (by contradiction method).

Solution:

(i) The given statement is as follows:

p: The sum of an irrational number and a rational number is irrational.

Let us assume that the statement p is false. That is,

The sum of an irrational number and a rational number is rational.

Hence,
where √a is irrational, and b, c, d, and e are integers.

∴ d / e – b / c = √a

But here, d / e – b / c is a rational number, and √a is an irrational number.

This is a contradiction. Hence, our assumption is false.

∴ The sum of an irrational number and a rational number is rational.

Hence, the given statement is true.

(ii) The given statement q is as follows:

If n is a real number with n > 3, then n² > 9,

Let us assume that n is a real number with n > 3, but n² > 9 is not true,

i.e., n² < 9

So, n > 3 and n is a real number,

By squaring both sides, we get

n² > (3)²

This implies that n² > 9, which is a contradiction since we have assumed that n² < 9,

Therefore, the given statement is true, i.e. if n is a real number with n > 3, then n² > 9.

7. Write the following statement in five different ways, conveying the same meaning.

p: If a triangle is equiangular, then it is an obtuse-angled triangle.

Solution:

The given statement can be written in five different ways, as given below:

(i) A triangle is equiangular implies that it is an obtuse-angled triangle.

(ii) A triangle is equiangular only if the triangle is an obtuse-angled triangle.

(iii) For a triangle to be equiangular, it is necessary that the triangle is an obtuse-angled triangle.

(iv) For a triangle to be an obtuse-angled triangle, it is sufficient that the triangle is equiangular.

(v) If a triangle is not an obtuse-angled triangle, then the triangle is not equiangular.

NCERT Solutions for Class 11 Maths Chapter 14 Mathematical Reasoning

Below are the important topics covered in Class 11 Maths Chapter 14, Mathematical Reasoning of NCERT Solutions:

14.1 Introduction

This section introduces the concepts covered in the chapter about Mathematical Reasoning using illustrations, the process of reasoning, different kinds of reasoning, and the fundamentals of deductive reasoning.

Mathematical Statement: Pythagorean Theorem holds good for any right-angled triangle.

Reasoning: If triangle ABC is a right triangle, it will follow Pythagorean Theorem.

14.2 Statements

This section defines a mathematical statement, a mathematically acceptable statement with examples.

Statement: Men are more intelligent than women.

Mathematical Statement: The product of two negative numbers is positive.

14.3 New Statements from Old

This section explains the production of new statements from the old ones. A technique is used in this process.

14.3.1 Negation of a Statement

This section covers the negation of a statement with a few solved examples.

Mumbai is a big city.

The negation of this statement will be:

1. Mumbai is not a big city.

2. It is false that Mumbai is a big city.

14.3.2 Compound Statements

This section talks about the compound statements obtained by using words like “and”, “or”, etc., including solved problems.

There is something wrong with the taste of the food or the vegetables being uncooked.

The above sentence consists of two smaller statements which are as follows:

There is something wrong with the taste of the food.

There is something wrong with the vegetables being uncooked.

14.4 Special Words/Phrases

This section defines connectives “and”, “or”, etc.

14.4.1 The Word “And”

81 is divisible by 3, 9 and 27.

The above statement has 3 small statements.

81 is divisible by 3.

81 is divisible by 9.

81 is divisible by 27.

14.4.2 The Word “Or”

A student who has taken Mathematics or Statistics can apply for the M.Sc Statistics programme.

The above statement means that students who have taken both Mathematics and Statistics can apply for the programme, also the students who have taken only one of these subjects.

14.4.3 Quantifiers

This section discusses different types of quantifiers with few illustrations.

There exists a square whose sides are equal.

For all natural numbers, n, 5n is an odd number.

14.5 Implications

This section discusses different types of implications with few illustrations.

Connecting a person to a certain crime even if no evidence is found.

14.5.1 Contrapositive and Converse

This section talks about contrapositive and converse statements with a few examples.

If you are not a citizen of India, then it will be difficult to obtain a passport in India. [contrapositive statement]

If you have solved all the exercises in the textbook, then you will get excellent marks in the exam.

If you get excellent marks in the exam, then you have solved all the exercises in the textbook. [converse]

14.6 Validating Statements

This section explains the process of validating the statements with specific cases along with problems.

14.6.1 By Contradiction

This section contains information about contradiction, the method of verifying a contradiction, and counter-examples explained through a few illustrations.

A Few Points on Chapter 14 Mathematical Reasoning

• A mathematically acceptable statement is a sentence which is either true or false.
• The negation of a statement p: If p denotes a statement, then the negation of p is denoted by ∼p.
• A statement is a compound statement if it is made up of two or more smaller statements. The smaller statements are called component statements of the compound statement.
• The words “And”, “Or”, “There exists”, and “For every” exist in compound statements.
• The implications are “If ”, “only if ”, and “ if and only if ”.

• The contrapositive of a statement p ⇒ q is the statement ∼ q ⇒ ∼p. The converse of a statement p ⇒ q is the statement q ⇒ p. p ⇒ q together with its converse gives p if and only if q.
• The following methods are used to check the validity of statements: (i) direct method, (ii) contrapositive method, (iii) method of contradiction, and (iv) using a counter-example.

Key mathematical concepts are covered in the NCERT Solutions for Class 11 Maths to help students progress ahead in their areas of study. The NCERT syllabus ensures that the content covered is appropriate for the students to move ahead in their respective streams in the future. A student needs to understand the concept of Mathematical Reasoning, as it is the main topic in the question paper and carries more marks. Before solving real-world applications and problems, the concept has to be learned thoroughly.