PTĐTTNT
`x^12+x^2+1`
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\(\left(x+1\right)\left(x+4\right)\left(x+2\right)\left(x+3\right)+1\)
\(=\left(x^2+5x+4\right)\left(x^2+5x+6\right)+1\)
\(=\left(x^2+5x+4\right)^2+2\left(x^2+5x+4\right)+1\)
\(=\left(x^2+5x+4+1\right)^2\)
\(=\left(x^2+5x+5\right)^2\)
(x + 1)(x + 2)(x + 3)(x + 4) - 24
= [(x + 1)(x + 4)][(x + 2)(x + 3)] - 24
= (x2 + 4x + x +4)(x2 + 3x + 2x + 12) - 24
= (x2 + 5x + 4)(x2 + 5x + 12) - 24
Đặt t = x2 + 5x + 8
Ta có: x2 + 5x + 4 = x2 + 5x + 8 - 4 (1)
x2 + 5x + 12 = x2 + 5x + 8 + 4 (2)
Thay t = x2 + 5x + 8 vào (1) và (2), ta có:
⇒ (t - 4)(t + 4) - 24
= t2 - 16 - 24
= t2 - 40
= (t - \(\sqrt{40}\))(t + \(\sqrt{40}\))
= (x2 + 5x + 8 - \(\sqrt{40}\))(x2 + 5x + 8 + \(\sqrt{40}\))
a) \(x^2+7x+12\)
\(=x^2+3x+4x+12\)
\(=x\left(x+3\right)+4\left(x+3\right)\)
\(=\left(x+3\right)\left(x+4\right)\)
b) \(3x^2-5x+2\)
\(=3x^2-3x-2x+2\)
\(=3x\left(x-1\right)-2\left(x-1\right)\)
\(=\left(x-1\right)\left(3x-2\right)\)
\(-\left(x+2\right)+3\left(x^2-4\right)\)
\(=3\left(x-2\right)\left(x+2\right)-\left(x+2\right)\)
\(=\left(x+2\right)\left[3\left(x-2\right)-1\right]=\left(x+2\right)\left(3x-7\right)\)