9【m+2n】的2次方-【2m-n】的2次方
=[3(m+2n)]^2-(2m-n)^2
=[3(m+2n)-(2m-n)][3(m+2n)+(2m-n)]
=(3m+6n-2m+n)(3m+6n+2m-n)
=(m+7n)(5m+5n)
=5(m+7n)(m+n)

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