已知m的2次方+3mn=5,求5m的2次方-【5m的2次方-(2m的2次方-mn)-7mn-5]

题目

已知m的2次方+3mn=5,求5m的2次方-【5m的2次方-(2m的2次方-mn)-7mn-5]
问下，在下有点不懂
=2m^2+6mn+5
=2(m^2+3mn)+5
=2*5+5
这一部下来6MN合并后应多出个3,请问去哪了啊
随先回答并正确就采纳谁

答案

m^2+3mn=5
5m^2-[5m^2-(2m^2-mn)-7mn-5]
=5m^2-5m^2+(2m^2-mn)+7mn+5
=2m^2-mn+7mn+5
=2m^2+6mn+5
=2(m^2+3mn)+5
=2*5+5
=15