a^2-ab+2b^2=3 求2ab-2a^2-4b^2-7的值

问题描述:

a^2-ab+2b^2=3 求2ab-2a^2-4b^2-7的值

2ab-2a^2-4b^2-7
=2(ab-a^2-2b^2)-7
=-2(a^2-ab+2b^2)-7
=(-2)*3-7
=-6-7=-13
肯定对哦