5mn-{5mn+[2m的2次方n+（m的2次方n-3mn）]-2m的2次方n}
=5mn-{5mn+[2m^2n+m^2n-3mn]-2m^2n}
=5mn-{5mn+[3m^2n-3mn]-2m^2n}
=5mn-{5mn+3m^2n-3mn-2m^2n}
=5mn-{2mn+m^2n}
=5mn-2mn-m^2n
=3mn-m^2n