当a=1/2,b=-7/2 (-a2+3ab-1/2b2)-(-2a2+4ab-1/2b2)的值
问题描述：
当a=1/2,b=-7/2 (-a2+3ab-1/2b2)-(-2a2+4ab-1/2b2)的值
答
解由(-a2+3ab-1/2b2)-(-2a2+4ab-1/2b2)
=(-a2+3ab-1/2b2)+(2a2-4ab+1/2b2)
=-a2+3ab-1/2b2+2a2-4ab+1/2b2
=a2-ab
=（1/2)^2-（1/2)(-7/2)
=1/4+7/4
=2