A trigonometric identity represents a relationship that is always true for all admissible values in the domain.

For example, 

sec θ  =  1 / cos θ

is is true for all admissible values of θ. Hence, this is an identity. 


sin θ  =  1/2

is not an identity, because the relation fails when θ = 60°.

Identities enable us to simplify complicated expressions. They are the basic tools of trigonometry which are being used in solving trigonometric equations.

The most important part of working with identities, is to manipulate them with the help of a variety of techniques from algebra.

Let us recall the fundamental identities (Pythagorean identities) of trigonometry, namely,

sin2θ + cos2θ  =  1

sec2θ - tan2θ  =  1

csc2θ - cot2θ  =  1

Points to Remember

1. sin2θ is the commonly used notation for (sin θ)2, likewise for other trigonometric ratios.

2. sec2θ - tan2θ  =  1 is meaningless when θ  =  90°But still it is an identity and true for all values of θ for which sec θ and tan θ are defined. Thus, an identity is an equation that is true for all values of its domain values.

3. When we write

sin θ / (1 + cos θ),

we understand that the expression is valid for all values of θ for which (1 + cos θ)  ≠  0

Solved Problems

Problem 1 :

Prove that 

Solution :

Problem 2 :

Prove that 

(secA − cosecA) (1 + tanA + cotA) = tanAsecA − cotA cosecA

Solution :

From (i) and (ii), we get the required result.

Problem 3 : 

Eliminate θ from

a cos θ  =  b

c sin θ  =  d

where a, b, c, d are constants.

Solution :

a cos θ  =  b

Multiply each side by c. 

ac cos θ  =  bc

Square each side. 

(ac cos θ)2  =  (bc)2

a2c2 cos2θ  =  b2c2 -----(1)

c sin θ  =  d

Multiply each side by a. 

ac sin θ  =  ad

Square each side. 

(ac sin θ)2  =  (ad)2

a2c2 sin2θ  =  a2d-----(2)

Add (1) and (2). 

a2c2 cos2θ + a2c2 sin2θ  =  b2c+  a2d2

a2c2(cos2θ + sin2θ)  =  b2c2  +  a2d2

a2c2(1)  =  b2c2  +  a2d2

a2c2  =  b2c2  +  a2d2

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