第48页 - 启东中学作业本七年级数学苏科版宿迁专版

D

B

$m²-2m-3$

$x²-y²$

-6

$ \begin{aligned}解：原式&=x^2+3x-5x-15 \\ &=x^2-2x-15. \\ \end{aligned}$

$ \begin{aligned}解：原式&=2t^2+4t-t-2 \\ &=2t^2+3t-2. \\ \end{aligned}$

$ \begin{aligned}解：原式&=2x^2-2xy+3xy-3y^2 \\ &=2x^2+xy-3y^2. \\ \end{aligned}$

$ \begin{aligned}解：原式&=m^2-\frac{1}{3}mn+\frac{1}{2}mn- \frac{1}{6}n^2 \\ &=m^2+\frac{1}{6} mn- \frac{1}{6}n^2. \\ \end{aligned}$

（更多请查看作业精灵详解）

$ \begin{aligned}解：原式&=x^2-6x+7x-42-x^2+2x \\ &=3x-42. \\ \end{aligned}$

A

C