Formulae

Formulae

A list of important general trigonometric identities,

$\cos^2 \theta + \sin^2 \theta = 1$ $\sec^2 \theta = 1 + \tan^2 \theta$ $\csc^2 \theta = 1 + \cot^2 \theta$

$\cos(90^{\circ} - \theta) = \sin \theta$ $\sin(90^{\circ} - \theta) = \cos \theta$ $\tan(90^{\circ} - \theta) = \cot \theta$

Some more specific identities that relate to the following general diagram,

$c^2 = a^2 + b^2 - 2a b \cos C$ $\frac{a}{\sin A} = \frac{b}{\sin B} = \frac{c}{\sin C} = 2R$

$\cos(A-B) = \cos A \cos B + \sin A \sin B$ $\cos(A+B) = \cos A \cos B - \sin A \sin B$ $\sin(A-B) = \sin A \cos B - \cos A \sin B$ $\sin(A+B) = \sin A \cos B + \cos A \sin B$

$\cos 2A = \cos^2 A - \sin^2 A$ $\cos 2A = 2 \cos^2 A - 1$ $\cos 2A = 1 - 2 \sin^2 A$ $\sin 2A = 2 \sin A \cos A$ $\tan (A + B) = \frac{\tan A + \tan B}{1-\tan A \tan B}$ $\tan (A - B) = \frac{\tan A - \tan B}{1+\tan A \tan B}$ $\tan 2A = \frac{2 \tan A}{1-\tan^2 A}$

$\tan (A+B+C) = \frac{\tan A + \tan B + \tan C - \tan A \tan B \tan C}{1- \tan B \tan C - \tan C \tan A - \tan A \tan B}$

If $A+B+C=180^{\circ}$

• $\displaystyle \tan A + \tan B + \tan C = \tan A \tan B \tan C$
• $\displaystyle \cot \tfrac{1}{2}A + \cot \tfrac{1}{2} B + \cot \tfrac{1}{2} C = \cot \tfrac{1}{2} A \cot \tfrac{1}{2} B \cot \tfrac{1}{2} C$
• $\displaystyle \sin A + \sin B + \sin C = 4 \cos \tfrac{1}{2} A \cos \tfrac{1}{2} B \cos \tfrac{1}{2} C$
• $\displaystyle \cos A + \cos B + \cos C -1 = 4 \sin \tfrac{1}{2} A \sin \tfrac{1}{2} B \sin \tfrac{1}{2} C$

$1+ \cos A = 2 \cos^2(\frac{1}{2}A)$ $1- \cos A = 2 \sin^2(\frac{1}{2}A)$

$\sin 2A = \frac{2 \tan A}{1+\tan^2 A}$ $\cos 2A = \frac{1- \tan^2 A}{1+\tan^2 A}$

$\sin X + \sin Y = 2 \sin \frac{X+Y}{2} \cos \frac{X-Y}{2}$ $\sin X - \sin Y = 2 \cos \frac{X+Y}{2} \sin \frac{X-Y}{2}$ $\cos X + \cos Y = 2 \cos \frac{X+Y}{2} \cos \frac{X-Y}{2}$ $\cos X - \cos Y = 2 \sin \frac{X+Y}{2} \sin \frac{Y-X}{2}$

$\frac{a-b}{a+b} \cot \frac{C}{2} = \tan \tfrac{1}{2}(A-B)$ $c = a \cos B + b \cos A$ $\arctan x + \arctan y = \arctan \frac{x+y}{1-xy}$ $\arctan x - \arctan y = \arctan \frac{x-y}{1+xy}$

where

• R stands for the circum-radius of the triangle ABC

To avoid doubt (and for those new to maths):

• $\cot \theta = 1 / \tan \theta$
• $\csc \theta = 1 / \sin \theta$
• $\sec \theta = 1 / \cos \theta$