Compute the gradient of the following:

a. \(f(x, y)=x^{2}-y^{2}\).

b. \(f(x, y, z)=y z+x y+x z\).

c. \(f(x, y)=\tan ^{-1}\left(\frac{y}{x}\right)\).

d. \(f(x, y, z)=2 y^{x} \cos z-5 \sin z \cos y\).