find the points of differentiability of f(z) where : 1) f(z) = $\displaystyle 4 x{^3}+ iy{^3} $ 2) f(z) = $\displaystyle x{^2} + y{^2}+ i2xy $ 3) f(z) = $\displaystyle xy + x{^2}yi $ 4) f(z) = $\displaystyle x{^3} + 3xy{^2}-3x+ i(y{^3}+3x{^2}y-3y) $ my solutions : 1) nowhere analytic and diff at y= $\displaystyle \pm 2x$ 2)nowhere analytic and diff at y=0 (x Axis) 3)nowhere analytic 4) x=0 or y=0