4[cd + ab]^2 - [a^2 + b^2 - c^2 - d^2]^2 = // a^2 - b^2 = (a - b) × (a + b) // =
= [2(cd + ab) - (a^2 + b^2 - c^2 - d^2)] × [2(cd + ab) + (a^2 + b^2 - c^2 - d^2)] =
= [c^2 + 2cd + d^2 - (a^2 - 2ab + b^2)] × [a^2 + 2ab + b^2 - (c^2 + 2cd +d^2)] = // a^2 + 2ab + b^2 = (a + b)^2 // =
= [(c + d)^2 - (a - b)^2] × [(a + b)^2 - (c - d)^2] =
= (c + d + a - b) × (c + d - a + b) × (a + b + c - d) × (a + b - c + d) =
= // a + b + c + d = 2p // =
=(2p - 2b) × (2p - 2a) × (2p - 2d) × (2p - 2c) = 16(p - a)(p - b)(p - c)(p - d)