This exercise contains problems based on all the concepts discussed in the chapter. Students solving these problems will acquire strong conceptual knowledge. The Selina Solutions for Class 10 Maths is the right place for students to clear their doubts and also learn the right steps to solve problems. The solutions to the Concise Selina Solutions for Class 10 Maths Chapter 21 Trigonometrical Identities Exercise 21(E) are available in a PDF, provided in the links enclosed below.
Selina Solutions Concise Maths Class 10 Chapter 21 Trigonometrical Identities Exercise 21(E) Download PDF
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Access Selina Solutions Concise Maths Class 10 Chapter 21 Trigonometrical Identities Exercise 21(E)
1. Prove the following identities:
Solution:
(i) Taking LHS,
1/ (cos A + sin A) + 1/ (cos A – sin A)
= RHS
– Hence Proved
(ii) Taking LHS, cosec A – cot A
= RHS
– Hence Proved
(iii) Taking LHS, 1 – sin2 A/ (1 + cos A)
= RHS
– Hence Proved
(iv) Taking LHS,
(1 – cos A)/ sin A + sin A/ (1 – cos A)
= RHS
– Hence Proved
(v) Taking LHS, cot A/ (1 – tan A) + tan A/ (1 – cot A)
= RHS
– Hence Proved
(vi) Taking LHS, cos A/ (1 + sin A) + tan A
= RHS
– Hence Proved
(vii) Consider LHS,
= (sin A/(1 – cos A)) – cot A
We know that, cot A = cos A/sin A
So,
= (sin2 A – cos A + cos2 A)/(1 – cos A) sin A
= (1 – cos A)/(1 – cos A) sin A
= 1/sin A
= cosec A
(viii) Taking LHS, (sin A – cos A + 1)/ (sin A + cos A – 1)
= RHS
– Hence Proved
(ix) Taking LHS,
= RHS
– Hence Proved
(x) Taking LHS,
= RHS
– Hence Proved
(xi) Taking LHS,
= RHS
– Hence Proved
(xii) Taking LHS,
= RHS
– Hence Proved
(xiii) Taking LHS,
= RHS
– Hence Proved
(xiv) Taking LHS,
= RHS
– Hence Proved
(xv) Taking LHS,
sec4 A (1 – sin4 A) – 2 tan2 A
= sec4 A(1 – sin2 A) (1 + sin2 A) – 2 tan2 A
= sec4 A(cos2 A) (1 + sin2 A) – 2 tan2 A
= sec2 A + sin2 A/ cos2 A – 2 tan2 A
= sec2 A – tan2 A
= 1 = RHS
– Hence Proved
(xvi) cosec4 A(1 – cos4 A) – 2 cot2 A
= cosec4 A (1 – cos2 A) (1 + cos2 A) – 2 cot2 A
= cosec4 A (sin2 A) (1 + cos2 A) – 2 cot2 A
= cosec2 A (1 + cos2 A) – 2 cot2 A
= cosec2 A + cos2 A/sin2 A – 2 cot2 A
= cosec2 A + cot2 A – 2 cot2 A
= cosec2 A – cot2 A
= 1 = RHS
– Hence Proved
(xvii) (1 + tan A + sec A) (1 + cot A – cosec A)
= 1 + cot A – cosec A + tan A + 1 – sec A + sec A + cosec A – cosec A sec A
= 2 + cos A/sin A+ sin A/cos A – 1/(sin A cos A)
= 2 + (cos2 A + sin2 A)/ sin A cos A – 1/(sin A cos A)
= 2 + 1/(sin A cos A) – 1/(sin A cos A)
= 2 = RHS
– Hence Proved
2. If sin A + cos A = p
and sec A + cosec A = q, then prove that: q(p2 – 1) = 2p
Solution:
Taking the LHS, we have
q(p2 – 1) = (sec A + cosec A) [(sin A + cos A)2 – 1]
= (sec A + cosec A) [sin2 A + cos2 A + 2 sin A cos A – 1]
= (sec A + cosec A) [1 + 2 sin A cos A – 1]
= (sec A + cosec A) [2 sin A cos A]
= 2sin A + 2 cos A
= 2p
3. If x = a cos θ and y = b cot θ, show that:
a2/ x2 – b2/ y2 = 1
Solution:
Taking LHS,
a2/ x2 – b2/ y2
4. If sec A + tan A = p, show that:
sin A = (p2 – 1)/ (p2 + 1)
Solution:
Taking RHS, (p2 – 1)/ (p2 + 1)
5. If tan A = n tan B and sin A = m sin B, prove that:
cos2 A = m2 – 1/ n2 – 1
Solution:
Given,
tan A = n tan B
n = tan A/ tan B
And, sin A = m sin B
m = sin A/ sin B
Now, taking RHS and substitute for m and n
m2 – 1/ n2 – 1
6. (i) If 2 sin A – 1 = 0, show that:
sin 3A = 3 sin A – 4 sin3 A
(ii) If 4 cos2 A – 3 = 0, show that:
cos 3A = 4 cos2 A – 3 cos A
Solution:
(i) Given, 2 sin A – 1 = 0
So, sin A = ½
We know, sin 30o = 1/2
Hence, A = 30o
Now, taking LHS
sin 3A = sin 3(30o) = sin 30o = 1
RHS = 3 sin 30o – 4 sin3 30o = 3 (1/2) – 4 (1/2)3 = 3 – 4(1/8) = 3/2 – ½ = 1
Therefore, LHS = RHS
(ii) Given, 4 cos2 A – 3 = 0
4 cos2 A = 3
cos2 A = 3/4
cos A = √3/2
We know, cos 30o = √3/2
Hence, A = 30o
Now, taking
LHS = cos 3A = cos 3(30o) = cos 90o = 0
RHS = 4 cos3 A – 3 cos A = 4 cos3 30o – 3 cos 30o = 4 (√3/2)3 – 3 (√3/2)
= 4 (3√3/8) – 3√3/2
= 3√3/2 – 3√3/2
= 0
Therefore, LHS = RHS
7. Evaluate:
Solution:
(i)
= 2 (1)2 + 12 – 3
= 2 + 1 – 3 = 0
(ii)
= 1 + 1 = 2
(iii)
(iv) cos 40o cosec 50o + sin 50o sec 40o
= cos (90 – 50)o cosec 50o + sin (90 – 50)o sec 40o
= sin 50o cosec 50o + cos 40o sec 40o
= 1 + 1 = 2
(v) sin 27o sin 63o – cos 63o cos 27o
= sin (90 – 63)o sin 63o – cos 63o cos (90 – 63)o
= cos 63o sin 63o – cos 63o sin 63o
= 0
(vi)
(vii) 3 cos 80o cosec 10o + 2 cos 59o cosec 31o
= 3 cos (90 – 10)o cosec 10o + 2 cos (90 – 31)o cosec 31o
= 3 sin 10o cosec 10o + 2 sin 31o cosec 31o
= 3 + 2 = 5
(viii)
8. Prove that:
(i) tan (55o + x) = cot (35o – x)
(ii) sec (70o – θ) = cosec (20o + θ)
(iii) sin (28o + A) = cos (62o – A)
(iv) 1/ (1 + cos (90o – A)) + 1/(1 – cos (90o – A)) = 2 cosec2 (90o – A)
(v) 1/ (1 + sin (90o – A)) + 1/(1 – sin (90o – A)) = 2 sec2 (90o – A)
Solution:
(i) tan (55o + x) = tan [90o – (35o – x)] = cot (35o – x)
(ii) sec (70o – θ) = sec [90o – (20o + θ)] = cosec (20o + θ)
(iii) sin (28o + A) = sin [90o – (62o – A)] = cos (62o – A)
(iv)
(v)
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