Rút gọn các biểu thức sau :
a ) ( x - 3 )(x + 2) - (x - 7 )(x - 1)
= \(\left[x\left(x+2\right)-3\left(x+2\right)\right]-\left[\left(x-7\right)\left(x-1\right)\right]\)
\(=\left[x.x+x.2-3.x+3.2\right]-\left[\left(x-7\right)\left(x-1\right)\right]\)
\(=\left[x^2+2x-3x+6\right]-\left[\left(x-7\right)\left(x-1\right)\right]\)
\(=\left[x^2+2x-3x+6\right]-\left[x\left(x-1\right)-7\left(x-1\right)\right]\)
\(=\left[x^2+2x-3x+6\right]-\left[x.x-x.1-7.x-1.x\right]\)
\(=\left[x^2+2x-3x+6\right]-\left[x^2-x-7x-x\right]\)
\(=x^2+2x-3x+6-x^2+x+7x+x\)
\(=\left(x^2-x^2\right)+\left(2x-3x+x+7x+x\right)+6\)
\(=8x+6.\)
b ) ( x - 2 )(x2 - 5x + 1 ) - x(x2 + 11)
Làm tương tự bài a
