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\Huge\sf \textbf{A Pythagorean-style proof of the sine sum-of-angles formula}
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\begin{center}\large
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Area of rhombus $=1\sin(x+y)=(\sin x +\sin y)(\cos x+\cos y)\ -$ area of 4 triangles\\[10pt]
$=\sin x\cos x+\sin x\cos y+\sin y\cos x+\sin y\cos y-2\cdot\frac12\sin x\cos x-2\cdot\frac12\sin y\cos y$\\[10pt]
\Large\bf Therefore \boldmath{$\sin(x+y)=\sin x\cos y+\cos x\sin y$}
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\begin{center}\tiny 
Peter Jipsen --- Andrew Moshier --- Math Poster 2011 --- math.chapman.edu
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