a) \displaystyle \nabla f=(2x,3y^2+4y^3)
b) \displaystyle \nabla f=(\frac{y}{\sqrt{1-x^2y^2}},\frac{x}{\sqrt{1-x^2y^2}})
c) \displaystyle \nabla f=(y(1+\tan^2x,\tan x)
