Posted by Anonymous on Tuesday, November 13, 2012 at 2:06pm.
luckily, you have nice easy 3-4-5 and 5-12-13 triangles.
cosx = -4/5
siny = 5/13
sin(x+y) = (3/5)(-12/13) + (-4/5)(5/13) = -56/65
reasonable, since (x+y) will be in QIII or QIV
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