M = (x + 2)(x + 4)(x + 6)(x + 8) + 16
M = [(x + 2)(x + 8)][(x + 4)(x + 6)] + 16
M = (x^2 + 2x + 8x + 16)(x^2 + 4x + 6x + 24) + 16
M = (x^2 + 10x + 16)(x^2 + 10x + 24) + 16
Đặt t = x^2 + 10x + 20
M = (t - 4)(t + 4) + 16
M = t^2 - 16 + 16 = t^2
Vậy ta có đpcm