Trigonometry For Class 11

Trigonometry is one of the major topics in Maths subject. Trigonometry deals with the measurement of angles and sides of a triangle. Usually, trigonometry is considered for the right-angled triangle. Also, its functions are used to find out the length of the arc of a circle, which forms a section in the circle with radius and its center point.

If we break the word trigonometry, ‘Tri’ is a Greek word which means ‘Three’, ‘Gon’ means ‘length’ and ‘metry’ means ‘measurement’. So basically, trigonometry is a study of triangles, which has angles and lengths on its side. Trigonometry basics consist of sine, cosine and tangent functions. Trigonometry for class 11 contains trigonometric identities to solve complex problems in a simpler way.

Trigonometry Formulas

Here we will discuss trigonometry formulas for class 11 along with trigonometric functions of Sum and Difference of two angles and trigonometric equations.

Starting with the basics of Trigonometry formulas, for a right-angled triangle ABC having an angle θ, opposite to perpendicular, we can define trigonometric ratios as;

Sin θ = P/H

Cos θ = B/H

Tan θ =P/B

Cot θ = B/P

Sec θ = H/B

Cosec θ = H/P





Trigonometry Functions

Trigonometry functions are measured in terms of radian for a circle drawn in XY plane. Radian is nothing but the measure of an angle just like a degree. The difference between the degree and radian is;

Degree:If rotation from the initial side to terminal side is (1/360)th of revolution, then the angle is said to measure 1 degree.

1 degree=60minutes

1 minute=60 second

Radian: If an angle is subtended at the center by an arc of length ‘l’ and then the angle is measured as radian. Suppose \(\Theta\) is the angle formed at the center, then,

θ = Length of the arc/radius of the circle.

θ = l/r

Relation between Degree and Radian:

2π radian=360°


π radian=180°

Where π = 22/7

Table for Degree and Radian relation

Degree 30° 45° 60° 90° 180° 270° 360°
Radian π/6 π/4 π/3 π/2 π 3π/2

Earlier we have discussed of trigonometric ratios for a degree, here we will write the table in terms of radians.

Trigonometry Table

Angle 0 π/6 π/4 π/3 π/2 π 3π/2
sin θ 0 1/2 1/√2 √3/2 1 0 -1 0
cos θ 1 √3/2 1/√2 1/2 0 -1 0 1
tan θ 0 1/√3 1 √3 undefined 0 undefined 0

Sign of Trigonometric Functions

sin(-θ) = -sin θ

cos(-θ) = cos θ

tan(-θ) = -tan θ

cot(-θ) = -cot θ

sec(-θ) = sec θ

cosec(-θ) = -cosec θ

Trigonometric Functions of Sum and Product of two angles


sin (x+y)=sin x cos y + cos x sin y

sin (x-y)= sin x cos y – cos x sin y

cos (x+y)=cos x cos y – sin x sin y

cos (x-y)= cos x cos y + sin x sin y

sin (π/2 – x)= cos x

cos (π/2 – x)= sin x


tan (x+y)= (tan x + tan y) /(1−tan x tan y)

tan (x-y) = (tan x − tan y)/(1 + tan x tan y)

cot (x+y) = (cot x cot y −1)/(cot y + cot x)

cot(x-y) = (cot x cot y + 1)/( cot y − cot x)


cos 2x = cos2 x-sin2 x = 2cos2 x-1 = 1-2sin2 x =1-tan2 x/1+tan2 x

Sin 2x= 2sin x cos x= 2tan x/ 1+ tan2 x

Tan 2x=2 tan x/1-tan2 x


sin3x= 3 sin x – 4 sin3 x

cos 3x= 4 cosx – 3 cos x

tan 3x= [3tan x-tan3 x]/[1-3 tan2 x]


\(cos\ x+ cos\ y=2\ cos{(\frac{x+y}{2})}\ cos{(\frac{x-y}{2})}\\ cos\ x – cos\ y = -2\ sin{(\frac{x+y}{2})}\ sin{(\frac{x-y}{2})}\\ sin\ x + sin\ y = 2\ sin{(\frac{x+y}{2})}\ cos{(\frac{x-y}{2})}\\ sin\ x – sin\ y = 2\ cos{(\frac{x+y}{2})}\ sin{(\frac{x-y}{2})}\)


2 cos x cos y = cos (x+y) + cos (x-y)

2 sin x sin y = cos (x-y) – cos (x+y)

2 sin x cos y= sin (x+y) + sin (x-y)

2 cos x sin y = sin (x+y) – sin (x-y)

To solve the trigonometric questions for class 11, all these functions and formulas are used accordingly. By practicing those questions, you can memorize the formulas as well.

Example 1:

Prove that sin(x+y)/ sin(x−y) = (tan x + tan y)/(tan x–tan y)


We have

LHS= sin(x+y)/sin(x−y)

= (sin x cos y + cos x  sin y)/(sin x cos y − cos x sin y)

Dividing numerator and denominator by cos x cos y, we get

= (tan x + tan y)/(tan x–tan y) ———–Proved.

Example 2:

Find the value of cos (31π/3).


We know that the value of cos x repeats after the interval 2π.

Thus, cos (31π/3) = cos (10π + π/3)

= cos π/3 = 1/2

Learn more about trigonometry by downloading BYJU’S- The Learning App and get interactive videos.

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