Question

先化简再求值:(a-2b)+2(a+b)(a-b)-3(a-2b)²,其中a=-2,b=1

Answer 1

(a-2b)+2(a+b)(a-b)-3(a-2b)²
=(a-2b)+2(a^2-b^2)-3(a-2b)²
=(a-2b)(1-3(a-2b))+2(a^2-b^2)
把a=-2,b=1代入得:
(-2-2)(1-3(-2-2))+2((-2)^2-1)
-4×13+6
=-46

追问

啊啊啊打错了是
a(a-2b)+2(a+b)(a-b)-3(a-2b)²

追答

a(a-2b)+2(a+b)(a-b)-3(a-2b)²

=a(a-2b)+2(a^2-b^2)-3(a-2b)²
=(a-2b)（a-3(a-2b))+2(a^2-b^2)
=(a-2b)(2b-2a)+2(a^2-b^2)
把a=-2,b=1代入得：

（-2-2）（2-2（-2））+2（（-2）^2-1)
=-4×6+6
=-18

Answer 2

(a-2b)+2(a+b)(a-b)-3(a-2b)²
=a-2b+2(a²-b²)-3(a²-4ab+4b²)
=a-2b+2a²-2b²-3a²+12ab-12b²
=-a²+12ab-14b²+a-2b
=-4+12x(-2)x1-14+(-2)-2
=-4-24-14-2-2
=-46

追问

啊啊啊打错了是
a(a-2b)+2(a+b)(a-b)-3(a-2b)²

追答

a(a-2b)+2(a+b)(a-b)-3(a-2b)²

=a²-2ab+2(a²-b²)-3(a²-4ab+4b²)

=a²-2ab+2a²-2b²-3a²+12ab-12b²
=10ab-14b²
=10x(-2)x1-14
=-20-14
=-34