Question

cho A =(x-2)(x^2+2x+4)-(x+1)^3+3(x-1)(x+1)

a/rút gọn A

b/tìm x để |A|=A

c/tìm x để x.A=-3x^2+2

Answer 1

a/ \(A=\left(x-2\right)\left(x^2+2x+4\right)-\left(x+1\right)^3+3\left(x-1\right)\left(x+1\right)\)

\(A=x^3+8-\left[x^3+1+3x\left(x+1\right)\right]+3\left(x^2-1\right)\)

\(A=x^3+8-x^3-1-3x\left(x+1\right)+3x^2-3\)

\(A=-3x^2-3x+3x^2+4\)

\(A=4-3x\)

b/ Để \(\left|A\right|=A\)

=> \(A\ge0\)

<=> \(4-3x\ge0\)

<=> \(4\ge3x\)

<=> \(x\ge\frac{3}{4}\)

Vậy khi \(x\ge\frac{3}{4}\)thì \(\left|A\right|=A\).