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Prove that

2sinxsiny = cos(x-y) - cos(x+y)

Prove that
sin2x + sin2y = 2sin(x+y)cos(x-y)

1 answer

On the first, you have formulas for cos(a+-b). Use them on the right, and it will simplifiy quickly.

On the second, I wouldstart with the double angle formulas...on the left.
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