a)x(x+1)(x+2)(x+3)+1
= (x2 + 3x)(x2 + 3x + 2) + 1
Đặt x2 + 3x = t, ta có:
t(t + 2) + 1
= t2 + 2t + 1
= (t + 1)2
= (x2 + 3x)2
b)(x^2+x+1)(x^2+3x+1)+x^2
Đặt x2 + x + 1 = t, ta có:
t(t - 2x) + x2
= t2 - 2xt + x2
= (t - x)2
= (x2 + x + 1 - x)2
= (x2 + 1)2