Wolfram Demonstrations Project

The double-angle formula for sine states that

. Computing the area of a triangle using the formula

(area equals one-half base times height) gives

for the top triangle and

for the bottom triangle.

This image is from J. D. Currie, "Proof without Words: Double Angle Formula via Area," Mathematics Magazine, 81(1), 2008 p. 62.

Contributed by: Chris Boucher

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A Visual Proof of the Double-Angle Formula for Sine