diffentiation

1.find the second derivative if 3x^2 + 2xy^2 = 1
Ans = (3y/4x^2) - (9/4y^3)
2. Let y= Asinx + Bcosx, where A and B are non-zero constants.
dy/dx = Acosx - Bsinx
d2y/dx2 = -Asinx- Bcosx
If 3(d2y/dx2) - 2(dy/dx) +6y = -13cosx, find A and B.
Ans : A=2,B= -3
3.y = sin(x+y)
find dy/dx.
hence show that [1-cos(x+y)](d2y/dx2) + y[1+dy/dx]^2=0