Double-Angle and Half-Angle Formulas

In this lesson, we will look at how to use the double angle formula and the half-angle formula.

The Double-Angle and Half-Angle Formulas are as follows:

Double Angle Formulas

sin 2a = 2 sin a cos a

cos 2a = cos 2a – sin 2a = 2 cos 2a – 1 =  1 – 2 sin 2a

Half Angle Formulas

Example:

Given , find
a) sin 2θ
b)

Solution:

a) sin 2θ = 2 sin θ cos θ

 

Videos

Solving Trigonometric Equations Using double-angle identities - Professor Edward Burger explains using double-angle identities

A few examples that use double-angle formulas from trigonometry.

Using half-angle identities - Professor Edward Burger explains using half-angle identities to solve a trigonometric equation

A few problems involving half-angle formulas in trigonometry.

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