Let `z_1=r_1(costheta_1+isintheta_1)a n dz_2=r_2(costheta_2+isintheta_2)` be two complex numbers. Then prove that `|z_1+z_2|^2=r1 2+r2 2+2r_1r_2cos(theta_1-theta_2)` or `|z_1+z_2|^2=|z_1|^2+|z_2|^2+2|z_1||z_2|^()_cos(theta_1-theta_2)` `|z_1-z_2|^2=r1 2+r2 2-2r_1r_2cos(theta_1-theta_2)` or `|z_1-z_2|^2=|z_1|^2+|z_2|^2-2|z_1||z_2|^()_cos(theta_1-theta_2)`