Polar form of a complex number z = x + iy is
z = r[ cos(theta) + i sin(theta) ]
and cos(theta) + i sin(theta) = e^i(theta)
again from polar for of z dz = [ cos(theta) + i sin(theta) ] dr + r [-sin(theta) + i cos(theta)] d(theta)
dz = [ cos(theta) + i sin(theta) ] dr + i r [ cos(theta) + i sin(theta)] d(theta) that gives required result