Since, siny=xsin(a+y) we have:
x=sinysin(a+y)
Deriving both sides we have:
dx=cosysin(a+y)−sin(y)cos(a+y)sin2(a+y)dy
dxdy=cos(y)sin(a+y)−sin(y)cos(a+y)sin2(a+y)
Using sin(a+b)=sin(a)cos(b)+sin(b)cos(a), we get that:
dxdy=sin(a+y−y)sin2(a+y)=sin(a)sin2(a+y)
So, dxdy=sin2(a+y)sin(a)