A Visual Proof of the Double-Angle Formula for Sine

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.


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