第68页 - 经纶学典学霸七年级数学上下册【江苏国标】

A

D

C

(x+5y)(x-5y)

(a+4)(a-4)

(2a+b)(2a-5b)

8

12

$解：原式=(2a+3)(2a-3).$

$解：原式=(\frac{1}{5}m+4n)(-\frac{1}{5}m+4n).$

B

C

B

3m+6

$ \begin{aligned} 解：(3)原式&=(3x-y+x-3y)(3x-y-x+3y) \\ &=(4x-4y)(2x+2y) \\ &=8(x- y)(x+y) \\ (4)原式&=(x²-7+2)(x²-7-2) \\ &=(x²-5)(x+3)(x-3) \\ \end{aligned}$

$ \begin{aligned} 解：原式&=a²+4ab-ab-4b²-3ab \\ &=a²-4b² \\ &=(a+2b)(a-2b) \\ (6)原式&=[2(a-b)+4(a+b)][2(a-b)-4(a+b)] \\ &=-4(3a+b)(a+3b) \\ \end{aligned}$

$ 解：(7)原式=(9a²+b²)(9a²-b²) $

$ ~~~~~~~~~~~~~~~~~~~~~~=(9a²+b²)(3a+b)(3a-b). $

$ \begin{aligned} (8)原式&=(3x+3y+3z-x+y+z)(3x+3y+3z+x-y-z) \\ &=(2x+4y+ 4z)(4x+2y+2z) \\ &=4(x+2y+2z)(2x+y+z) \\ \end{aligned}$