Question

tính

a/A=(x+1)^3-(x+3)^2(x+1)+4x^2+8 vs x=-1/6

b/B=(x-1)^3-(x+2)(x^2-2x+4)+3(x+4)(x-4) vs x=-2

Answer 1

\(A=\left(x+1\right)^3-\left(x+3\right)^2\left(x+1\right)+4x^2+8\)
\(A=\left(x^3+3x^2+3x+1\right)-\left(x^2+6x+9\right)\left(x+1\right)-4x^2+8\)
\(A=\left(x^3+3x^2+3x+1\right)-\left(x^3+x^2+6x^2+6x+9x+9\right)+4x^2+8\)
\(A=x^3+3x^2+3x+1-x^3-x^2-6x^2-6x-9x-9+4x^2+8\)
\(A=-12x\)
Thay \(x=-\dfrac{1}{6}\) vào \(A\) ta có:
\(A=-12\times\left(-\dfrac{1}{6}\right)=2\)
Vậy \(A=2\) khi \(x=-\dfrac{1}{6}\)

\(B=\left(x-1\right)^3-+\left(x+2\right)\left(x^2-2x+4\right)+3\left(x+4\right)\left(x-4\right)\)
\(B=\left(x^3-3x^2+3x-1\right)-\left(x^3-2x^2+4x+2x^2-4x+8\right)+\left(3x^2-48\right)\)
\(B=x^3-3x^2+3x-1-x^3+2x^2-4x-2x^2+4x-8+3x^2-48\)
\(B=3x-57\)
Thay \(x=-2\) vào \(B\) ta có:
\(B=3\times\left(-2\right)-57=-6-57=-63\)
Vậy \(B=-63\) khi \(x=-2\)