sina = x、sinb = y
cosa = ±√(1 - x^2)
cosb = ±√(1 - y^2)
sin(a + b)
= sina*cosb + cosa*sinb
= ±[√(1 - y^2)]*x ±[√(1 - x^2)]*y
sina=x sinb=y sin(a+b)=?
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