In these lessons, we will learn to use trigonometric identities, including cofunction identities, power reducing formulas and half-angle identities.

The following tables give some Trigonometric Identities. Scroll down the page for examples and solutions on how to use the Trig Identities. Using a Cofunction Identity
Cofunction identities and how to determine cofunctions given a function value.

Cofunction Identities - Solving Trigonometric Equations
How to use cofunction identities to solve trigonometric equations?

Example:
Find a possible acute angle solution.
a) cos(2θ + 16°) = sin(θ + 11°)
b) cot(θ) = tan(θ + π/6)

Power Reducing Formulas - Trigonometric Identities
How to use power reducing formulas to simplify trigonometric expressions?
It contains the power reducing trigonometric identities for sine, cosine, and tangent.

Examples:
sin4(x)
sin2(x)cos2(x)
sin4(x)cos2(x).

Half Angle Identities to Evaluate Trigonometric Expressions
Find the exact value of the following:
tan(105°)

How to use Half Angle Identities to Evaluate Trigonometric Expressions
Find the exact value of the following:
sin(a/2) if cos a = 3/5 for 0° ≤ a ≤ 90°

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