# Half Angle Identities

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Videos, worksheets, games and activities to help PreCalculus students learn how to derive and use the half angle identities.

Using Half-Angle Identities to Solve a Trigonometric Equation
Half Angle Identities to Evaluate Trigonometric Expressions, Example 1.
This video gives some half angle identities and show how they can be used to solve some trigonometric equations.
Half Angle Identities to Evaluate Trigonometric Expressions, Example 2.
This video gives some half angle identities and show how they can be used to solve some trigonometric equations.

Half Angle Identities to Evaluate Trigonometric Expressions, Example 3.
This video gives some half angle identities and show how they can be used to solve some trigonometric equations.
Solving Trigonometric Equations: Half-Angle Identities

This video shows how half-angle identitied to determine function values.
A few problems involving half-angle formulas in trigonometry.

The derivations of the half-angle identities for both sine and cosine, plus listing the tangent ones. Then a couple of examples using the identities.
This video derives the half angle trigonometry identities for cosine, sine and tangent

The Half-Angle Identities : The half angle identities come from the power reduction formulas using the key substitution alpha = theta/2 twice, once on the left and right sides of the equation. With half angle identies, on the left side, this yields (after a square root) cos(theta/2) or sin(theta/2); on the right side cos(2alpha) becomes cos(theta) because 2(1/2) = 1. For a problem like sin(pi/12), remember that theta/2 = pi/12, or theta = pi/6, when substituting into the identity.
How to use the power reduction formulas to derive the half-angle formulas.

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