先化简再求值(a-2b)(3a-2b)-3(a-b)(a+3b),其中a=-(1/2),b=1
问题描述：
先化简再求值(a-2b)(3a-2b)-3(a-b)(a+3b),其中a=-(1/2),b=1
答
(a-2b)(3a-2b)-3(a-b)(a+3b)
=3a²-2ab-6ab+4b²-3(a²+3ab-ab-3b²)
=3a²-8ab+4b²-3(a²+2ab-3b²)
=3a²-8ab+4b²-3a²-6ab+9b²
=13b²-14ab
=b(13b-14a)
=1X(13X1-14X(-1/2))
=13+7
=20