Question

一道计算题 先化简，后求值

-2（mn-3m²)-｛m²-5(mn-m²)+2mn｝，其中m=1，n=2

Answer 1

你好，Fate木棉

先化简：
-2(mn-3m²)-[m²-5(mn-m²)+2mn]
=-2mn+6m²-(m²-5mn+5m²+2mn)
=-2mn+6m²-(6m²-3mn)
=-2mn+6m²-6m²+3mn
=mn
∴当m=1，n=2时
原式=1×2=2

Answer 2

-2（mn-3m²)-｛m²-5(mn-m²)+2mn｝
=-2mn+6m²-m²+5(mn-m²)-2mn
=-4mn+5m²+5mn-5m²
=mn
=2

Answer 3

-2（mn-3m²)-｛m²-5(mn-m²)+2mn｝
=mn=2