因为|m-n-2|大于等于0 (mn-1）的2次方大于等于0且|m-n-2|与(mn-1）的2次方互为相反数
所以m-n-2=0 mn-1=0
则m-n=2 mn=1
(-2mn+2m+3n)-(3mn+2n-2m)-(m+4n+mn)
=-6mn+3m-3n
=3(-2mn+m-n)
将m-n=2 mn=1代入,得原式=3*（-2*1+2）=0