m+n=5,m²+n²=17
(m+n)^2=m^2+n^2+2mn=25
mn=4
m²n-3m²n²+mn²
=mn(m+n)-3(mn)^2
=5mn-3(mn)^2
=20-3*16
=-28(m+n)^2=m^2+n^2+2mn=25mn=4这步没看懂详细解释下(m+n)^2=m^2+n^2+2mn (m²+n²=17)=17+2mn=25mn=4