8A:
a:Đặt \(A=\left(x+1\right)\left(x^2+2x+4\right)-x^2\left(x+3\right)\)
\(=x^3+2x^2+4x+x^2+2x+4-x^3-3x^2\)
=6x+4
Khi x=-10/3 thì \(A=6\cdot\dfrac{-10}{3}+4=-20+4=-16\)
b: Đặt \(A=4\left(\dfrac{3}{4}x-1\right)+\dfrac{\left(12x^2-3x\right)}{-3x}-\left(2x-1\right)\)
\(=3x-4-4x+1-2x+1\)
=-3x-2
Khi x=3 thì \(A=-3\cdot3-2=-11\)
8B:
a: Đặt \(A=\left(x-3\right)^2-\left(8x+3\right)\left(3-x\right)+x\left(x-3\right)\)
\(=\left(x-3\right)^2+\left(8x+3\right)\left(x-3\right)+x\left(x-3\right)\)
\(=\left(x-3\right)\left(x-3+8x+3+x\right)\)
\(=\left(x-3\right)\cdot10x\)
Khi x=103 thì \(A=10\cdot103\cdot\left(103-3\right)=10\cdot100\cdot103=103000\)
b: Đặt A=\(5\left(-\dfrac{3}{5}x+2\right)+\left(5x^3-3x^2\right):\left(-2x\right)^2+x-3\)
\(=-3x+10+\dfrac{5x^3-3x^2}{4x^2}+x-3\)
\(=-2x+7+\dfrac{5}{4}x-\dfrac{3}{4}\)
\(=-\dfrac{3}{4}x+\dfrac{25}{4}\)
Khi x=-3 thì \(A=-\dfrac{3}{4}\cdot\left(-3\right)+\dfrac{25}{4}=\dfrac{25}{4}+\dfrac{9}{4}=\dfrac{34}{4}=\dfrac{17}{2}\)
