已知x+y=3,x²+y²-3xy=4, yx^3+xy^3=_____

已知x+y=3,x²+y²-3xy=4, yx^3+xy^3=_____
已知x+y=3,x²+y²-3xy=4, yx^3+xy^3=_____

已知x+y=3,x²+y²-3xy=4, yx^3+xy^3=_____
x+y=3
(x+y)²=9
x²+2xy+y²=9
又已知x²+y²-3xy=4
相减,得:
5xy=5
xy=1
所以x²+y²=9-2=7
yx³+xy³
=xy(x²+y²)
=7