Bài 7
\(D\left(x\right)=\left(x-2\right)\left(2x^2+1\right)-\left(2x+2\right)\left(x^2-3x\right)\)
\(=2x^3+x-4x^2-2-2x^3+6x^2-2x^2+6x\)
\(=\left(2x^3-2x^3\right)+\left(-4x^2+6x^2-2x^2\right)+\left(x+6x\right)-2\)
\(=7x-2\)
Tại \(x=2\Rightarrow D\left(2\right)=7.2-2=12\)
Bài 8
\(E\left(x\right)=\left(2x+5\right)\left(1+x^2\right)-\left(-2x+x^2\right)\left(-1+2x\right)\)
\(=2x+2x^3+5+5x^2-2x+4x^2+x^2-2x^3\)
\(=\left(2x^3-2x^3\right)+\left(5x^2+4x^2+x^2\right)+\left(2x-2x\right)+5\)
\(=10x^2+5\)
Tại \(x=-5\Rightarrow E\left(-5\right)=10.\left(-5\right)^2+5=255\)
Bài 9
\(A\left(x\right)=x^{2m-1}.\left(x^{2-2m}-x^{1-2m}\right)\)
\(=x^{2m-1+2-2m}-x^{2m-1+1-2m}\)
\(=x-x^0\)
\(=x-1\)
Tại \(x=20\Rightarrow A\left(20\right)=20-1=19\)