x²+mx+n=(x-3)(x+4)
x²+mx+n=x²+x-12
(m-1)x+(n+12)=0
要对任意x,等式恒成立,只有
m-1=0
n+12=0
解得m=1 n=-12
m²n+mn²
=mn(m+n)
=1×(-12)×[1+(-12)]
=132