(x+my)(x+ny)=x^2+2xy-8y^2
(x+my)(x+ny)
=x^2+(m+n)xy+mny^2
=x^2+2xy-8y^2
根据对应项系数相等,得
m+n=2
mn=-8
解得
m=4
n=-2
mn(m+n)=-8×2=-16