Trigonometric Identities
In this minilesson, we will explore trigonometric identities. We have already learned the primary trigonometric ratios sin, cos, and tan. We have also learned that there are three other trigonometric ratios sec, cosec, and cot which are the reciprocal of sin, cos, and tan respectively. How are these trigonometric ratios (sin, cos, tan, sec, cosec, and cot) connected with each other? They are connected through trigonometric identities (or in short trig identities). Let's start learning about trigonometric identities.
Trigonometric Identities
Trigonometric identities are equations that relate different trigonometric functions and are true for any value of the variable that is there in the domain. Basically, an identity is an equation that holds true for all the values of the variable(s) present in it.
For example, some of the algebraic identities are:
(a+b)^{2 }= a^{2 }+ 2ab + b^{2}
(ab)^{2 }= a^{2 } 2ab+ b^{2}
(a+b)(ab)= a^{2 } b^{2}
The algebraic identities relate just the variables whereas the trig identities relate the 6 trigonometric functions sine, cosine, tangent, cosecant, secant, and cotangent. Let's learn about each type of identities in detail.
Reciprocal Identity
We already know that the reciprocals of sin, cosine, and tangent are cosecant, secant, and cotangent respectively.
Thus, the reciprocal identities are:
 sin θ = 1/cosecθ (OR) cosec θ = 1/sinθ
 cos θ = 1/secθ (OR) sec θ = 1/cosθ
 tan θ = 1/cotθ (OR) cot θ = 1/tanθ
Pythagorean Identities
The Pythagorean identities in trigonometry are derived from the Pythagoras theorem. Applying Pythagoras theorem to the rightangled triangle below, we get:
Opposite^{2 }+ Adjacent^{2 }= Hypotenuse^{2}
Dividing both sides by Hypotenuse^{2}
Opposite^{2}/Hypotenuse^{2} + Adjacent^{2}/Hypotenuse^{2} = Hypotenuse^{2}/Hypotenuse^{2}
 sin^{2}θ + cos^{2}θ = 1
This is one of the Pythagorean identities. In the same way, we can derive two other Pythagorean identities.
 1+tan^{2}θ = sec^{2}θ
 1+cot^{2}θ = cosec^{2}θ
Complement and Supplement of a Ratio
The complementary angles are a pair of two angles such that their sum is equal to 90°. The complement of an angle θ is (90  θ). The trigonometric ratios of complementary angles are:
 sin (90° θ) = cos θ
 cos (90° θ) = sin θ
 cosec (90° θ) = sec θ
 sec (90° θ) = cosec θ
 tan (90° θ) = cot θ
 cot (90° θ) = tan θ
The supplementary angles are a pair of two angles such that their sum is equal to 180°. The supplement of an angle θ is (180  θ). The trigonometric ratios of supplementary angles are:
 sin (180° θ) = sinθ
 cos (180° θ) = cos θ
 cosec (180° θ) = cosec θ
 sec (180° θ)= sec θ
 tan (180° θ) = tan θ
 cot (180° θ) = cot θ
Sum and Difference Formulas
The sum and difference identities include the formulas of sin(A+B), cos(AB), cot(A+B), etc.
 sin (A+B) = sin A cos B + cos A sin B
 sin (AB) = sin A cos B  cos A sin B
 cos (A+B) = cos A cos B  sin A sin B
 cos (AB) = cos A cos B +sin A sin B
 tan (A+B) = (tan A + tan B)/ (1  tan A tan B)
 tan (AB) = (tan A  tan B)/ (1 + tan A tan B)
Double and Half Angles
Double angle formulas: The double angle identities can be obtained by using the sum and difference formulas.
For example, from the above formulas:
sin (A+B) = sin A cos B + cos A sin B
Substitute A = B = θ on both sides here, we get:
sin (θ + θ) = sinθ cosθ + cosθ sinθ
sin 2θ = 2 sinθ cosθ
In the same way, we can derive the other double angle identities.
 sin 2θ = 2 sinθ cosθ
 cos 2θ = cos^{2}θ  sin^{2}θ
= 2 cos^{2}θ  1
= 1  2 sin^{2}θ  tan 2θ = (2 tanθ)/ (1  tan^{2}θ)
Half Angle Formulas
Using one of the above double angle formulas,
cos 2θ = 1  2 sin^{2}θ
2 sin^{2}θ = 1 cos 2θ
sin^{2}θ = (1cos2θ)/(2)
sin θ = \(\pm \sqrt{\dfrac{1cos 2θ}{2}}\)
Replacing θ by θ/2 on both sides,
\[sin \frac{θ}{2}=\pm \sqrt{\frac{1cos θ}{2}}\]
This is the halfangle formula of sin.
In the same way, we can derive the other halfangle formulas.
\[\begin{array}{l}1.\,\,sin \dfrac{θ}{2}=\pm \sqrt{\dfrac{1cos θ}{2}} \\[0.2cm]
2. \,\, cos \dfrac{θ}{2}=\pm \sqrt{\dfrac{1+cos θ}{2}} \\[0.2cm]
3. \,\,\tan \dfrac{θ}{2}=\pm \sqrt{\dfrac{1cos θ}{1+cos θ}}
\end{array}\]
The trigonometric identities that we have learned are derived using the rightangled triangles. There are a few other identities that we use in the case of the triangles that are not rightangled.
Sine and Cosine Rule
The sine rule gives the relation between the angles and the corresponding sides of a triangle. For the nonrightangled triangles, we will have to use the sine rule and the cosine rule.
The cosine rule gives the relation between the angles and the sides of a triangle and is usually used when two sides and the included angle of a triangle are given.
Related Articles:
 Trigonometric Table
 Trigonometric Formula
 Trigonometry
 Inverse Trigonometric Formulas
 Sum and Difference Identities Calculator
Important Notes
 To write the trigonometric ratios of complementary angles, we consider the following as pairs: (sin, cos), (cosec, sec), and (tan, cot).
 While writing the trigonometric ratios of supplementary angles, the trigonometric ratio won't change. The sign can be decided using the fact that only sin and cosec are positive in the second quadrant where the angle is of the form (180θ).
 There are 3 formulas for the cos 2x formula. Among them, you can remember just the first one because the other two can be obtained by the Pythagorean identity sin^{2} x + cos^{2}x = 1.
 The halfangle formula of tan is obtained by applying the identity tan = sin/cos and then using the halfangle formulas of sin and cos.
Solved Examples

Example 1: Can we help Tina prove the following identity using the trig identities?
\(\frac{sin 3 θ+cos 3 θ}{sin θ+cos θ}+sin θ cos θ=1\)
Solution:
We make use of the following identity:
a^{3}+b^{3 }= (a+b)(a^{2}ab+b^{2})
We use the Pythagorean identities to prove this identity.
\(\begin{align}
&L H S\\[0.2cm] &=\frac{sin 3 θ+cos 3 θ}{sin θ+cos θ}+sin θ cos θ \\[0.2cm]&=\frac{(sin θ+cos θ)\left(sin^ 2 θsin θ cos θ+cos^ 2 θ\right)}{sin θ+cos θ}+sin θ cos θ \\[0.2cm]
&=\left(sin ^2 θsin θ cos θ+cos ^2 θ\right)+sin θ cos θ \\[0.2cm]
&=sin^2 θ+cos^2 θ\\[0.2cm]&=1\\[0.2cm]&=R H S
\end{align}\) 
Example 2:
Can we help Jake prove the following identity using the trig identities?
(sin θ+cosec θ)^{2}+(cos θ+sec θ)^{2}=7+tan^{2} θ+cot^{2} θ
Solution:
We use the reciprocal identities and Pythagorean identities to prove this identity.
L H S
= (sin^{2} θ + cosec^{2} θ + 2 sin θ cosec θ) + (cos^{2} θ + sec^{2} θ + 2 cos θ sec θ)
=sin^{2} θ + cos^{2} θ + cosec^{2}θ + sec^{2}θ + 2 + 2
=1 + (1+cot^{2} θ) + (1+tan^{2} θ) + 2 + 2
=7 + tan^{2} θ + cot^{2} θ = R H S 
Example 3:
Can we help James find the exact value of sin75°?
Solution:
We know that 75° = 30°+45°
We apply the sum formula of sin to find the value of sin 75°.
sin (A+B) = sin A cos B + cos A sin B
Substitute A=30° and B=45° here on both sides:
sin (30° + 45°) = sin 30° cos 45°+ cos 30° + sin 45°
sin 75° = \(\dfrac{1}{2}\cdot \dfrac{\sqrt{2}}{2}+ \dfrac{\sqrt{3}}{2} \cdot \dfrac{\sqrt{2}}{2}\)
sin 75° = \(\dfrac{\sqrt{2}+ \sqrt{6}}{4}\) =\( \dfrac{\sqrt3 + 1}{2\sqrt2} \)Here, the values of sin 30°, cos 45°, cos 30°, and sin 45° can be obtained by using the trigonometric table.
Answer: Sin75°= \( \dfrac{\sqrt3 + 1}{2\sqrt2} \)
FAQs on Trigonometric Identities
What Are the 3 Trigonometric Identities?
The three trigonometric identities are:
 sin^{2}θ + cos^{2}θ = 1
 1+tan^{2}θ = sec^{2}θ
 1+cot^{2}θ = cosec^{2}θ
How Many Identities Are There in Trigonometry?
The 36 Trig Identities, some of the trigonometric identities are key for you to know, and others will be used rarely or never.
What Is the Trigonometry Formula?
Basic trigonometric function formulas are:
Sin θ = Opposite Side/Hypotenuse, Cos θ = Adjacent Side/Hypotenuse and Tan θ = Opposite Side/Adjacent Side.
Do I Need To Memorize Trig Identities?
Try to derive the trigonometric identities using the procedures that are explained on this page. If you can derive them on your own, then memorizing them will not be a problem.
What Trig Identities We Must Know Definitely?
All trig identities are used in solving the problems. The main trig identities are Pythagorean identities, reciprocal identities, sum and difference identities, double angle and halfangle identities. For the nonrightangled triangles, we will have to use the sine rule and the cosine rule.
What Are Eight Fundamental Trigonometric Identities?
The eight fundamental trigonometric identities are:
 cosec θ = 1/sin θ
 sec θ = 1/cos θ
 cot θ = 1/ tan θ
 sin^{2}θ + cos^{2}θ = 1
 tanθ = sinθ/ cos θ
 1+ tan^{2}θ = sec^{2}θ
 cot θ = cosθ/ sinθ
 1+ cot^{2}θ = cosec^{2}θ
When Do You Use the Double Angle Formula?
The double angle formulas are used when the angle is of form 2 multiplied by a variable0