Let arg z = x
/_ z(zw)O = x (alternate segment theorem)
/_ wO(zw) = x (w is rotated x degrees to zw)
/_ O(w)(z) = 2x (exterior angle in triangle equals sum of opp int angles)
/_ (w)(z)(z+w) = 2x (alternate angles are equal, opp sides of parallelogram are parallel)
/_ (zw)O(z+w) = 2x (...