• Algebra
• Trigonometry
• Geometry
• Calculus

Double angle identities

In trigonometry, there are four popular double angle trigonometric identities and they are used as formulae in theorems and in solving the problems. So, let’s learn each double angle identity with mathematical proof.

Sin Double angle identity

$\sin{2\theta} \,=\, 2\sin{\theta}\cos{\theta}$

Cos Double angle identity

$\cos{2\theta} \,=\, \cos^2{\theta}-\sin^2{\theta}$

Tan Double angle identity

$\tan{2\theta} \,=\, \dfrac{2\tan{\theta}}{1-\tan^2{\theta}}$

Cot Double angle identity

$\cot{2\theta} \,=\, \dfrac{\cot^2{\theta}-1}{2\cot{\theta}}$

Problems

Learn how to use the double angle trigonometric identities as formulae in mathematical problems.