x-y=3,
两边平方,
x²+y² -2xy=9
13-2xy=9
xy=2
(x+y)²=x²+y²+2xy=13+2×2=17
x+y=±根号17
x³y²+x²y³
=x²y²(x+y)
=(xy)²(x+y)
=2²×(±根号17)
=±4根号17