Đáp án:
$\displaystyle \begin{array}{{>{\displaystyle}l}} a.x^{3} y+4x^{2} y+xy\\ b.-5x^{4} y-25x^{3} y-5x^{2} y\\ c.\frac{2}{3} x^{2} yz-\frac{2}{3} xy^{2} z\\ d.8x^{2} -2\\ e.x^{2} +2x+y+xy+1\\ f.x^{2} -81 \end{array}$
Giải thích các bước giải:
$\displaystyle \begin{array}{{>{\displaystyle}l}} a.\ xy\left( x^{2} +4x+1\right) =x^{3} y+4x^{2} y+xy\\ b.\ -5x^{2} y\left( x^{2} -5x+1\right) =-5x^{4} y-25x^{3} y-5x^{2} y\\ c.\ \frac{2}{3} xyz( x-y) =\frac{2}{3} x^{2} yz-\frac{2}{3} xy^{2} z\\ d.\ ( 2x+1)( 4x-2) =8x^{2} -2\\ e.\ ( x+y+1)( x+1) =x^{2} +x+xy+y+x+1\\ =x^{2} +2x+y+xy+1\\ f.\ ( x-9)( x+9) =x^{2} -81 \end{array}$