Formula
$$z_1 \cdot z_2 = r_1\left(\cos\psi_1+i\sin\psi_1\right) \cdot r_2\left(\cos\psi_2+i\sin\psi_2\right) = \\ = r_1r_2\left(\cos\left(\psi_1+\psi_2\right)+i\sin\left(\psi_1+\psi_2\right) \right)$$
$$z_1 \cdot z_2 = r_1\left(\cos\psi_1+i\sin\psi_1\right) \cdot r_2\left(\cos\psi_2+i\sin\psi_2\right) = \\ = r_1r_2\left(\cos\left(\psi_1+\psi_2\right)+i\sin\left(\psi_1+\psi_2\right) \right)$$