phan tich thnah nhan tu
(x-1)(x+2)(x+3)(x+6)+5
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\(x^7+x^5+x^4+x^3+x^2+1\)
\(=\left(x^7+x^4\right)+\left(x^5+x^2\right)+\left(x^3+1\right)\)
\(=x^4\left(x^3+1\right)+x^2\left(x^3+1\right)+\left(x^3+1\right)\)
\(=\left(x^3+1\right)\left(x^4+x^2+1\right)\)
\(=\left(x+1\right)\left(x^2-x+1\right)\left(x^4+x^2+1\right)\)
a, k ph đc
b,Đặt \(A=...=\left(x+1\right)\left(x+4\right)\left(x+2\right)\left(x+3\right)-24=\left(x^2+5x+4\right)\left(x^2+5x+6\right)-24\)
Đặt x^2+5x+4=t,ta có:
\(A=t\left(t+2\right)-24=t^2+2t-24=t^2-4t+6t-24=t\left(t-4\right)+6\left(t-4\right)=\left(t-4\right)\left(t+6\right)\)
\(=\left(x^2+5x+4-4\right)\left(x^2+5x+4+6\right)=x\left(x+5\right)\left(x^2+5x+10\right)\)
\(\left(x-1\right)\left(x-3\right)\left(x-5\right)\left(x-7\right)-20=\left[\left(x-1\right)\left(x-7\right)\right].\left[\left(x-3\right)\left(x-5\right)\right]-20\)
\(=\left(x^2-8x+7\right)\left(x^2-8x+15\right)-20\)
Đặt \(x^2-8x+11=t\) \(\Rightarrow\left(x^2-8x+7\right)\left(x^2-8x+15\right)-20=\left(t-4\right)\left(t+4\right)-20=t^2-16-20=t^2-36=\left(t-6\right)\left(t+6\right)\)\(\Rightarrow\left(x-1\right)\left(x-3\right)\left(x-5\right)\left(x-7\right)-20=\left(x^2-8x+11-6\right)\left(x^2-8x+11+6\right)=\left(x^2-8x+17\right)\left(x^2-8x+5\right)\)
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