第143页 - 启东中学作业本七年级数学苏科版宿迁专版

C

A

D

B

±6

-5x

$a(a-3)²$

19

24和26

$ \begin{aligned} 解：原式&=3m^2-mn+9mn-3n^2-2(m^2-2mn+n^2) \\ &=3m^2+8mn-3n^2-2m^2+4mn-2n^2 \\ &=m^2+12mn-5n^2. \\ \end{aligned}$

$ \begin{aligned}解：原式&=(2x+y)²-2² \\ &=4x²+4xy+y²-4. \\ \end{aligned}$