(a-b)^2=a^2+b^2-2ab≥0
a^2+b^2≥2ab
M≥N
m^2+2mn-15n^2=(m+5n)(m-3n)
4（m+2n)^2-9(2n-m)^2
=(2m+4n)^2-(6n-3m)^2
=(2m+4n+6n-3m)(2m+4n-6n+3m)
=(10n-m)(5m-2n)m^2+2mn-15n^2=(m+5n)(m-3n)过程？十字相乘，要什么过程教我一下怎么相乘m5nm-3n-3mn+5mn=2mn(m+5n)(m-3n)