lim [f(a+h^2)-f(a)]/h=h*lim [f(a+h^2)-f(a)]/h^2=h*f'(a);
lim [f(a+3h)-f(a-h)]/2h=2*lim [f(a+3h)-f(a-h)]/(2h*2)= 2*f'(a);