已知A=a的平方—ab+2b的平方,B=—a的平方—b的平方,求A—2B的值
人气：140 ℃ 时间：2020-05-09 13:06:30
解答
A-2B
=(a^2-ab+2b^2)-2(-a^2-b^2)
=a^2-ab+2b^2+2a^2+2b^2
=3a^2-ab+4b^2
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