已知x²+xy=5,xy+y²=-3,代数式3x²+xy-2y²的值?

已知x²+xy=5,xy+y²=-3,代数式3x²+xy-2y²的值?
已知x²+xy=5,xy+y²=-3,代数式3x²+xy-2y²的值?

已知x²+xy=5,xy+y²=-3,代数式3x²+xy-2y²的值?
x²+xy=5,xy+y²=-3
3x²+xy-2y²
=3(x²+xy)-2xy-2y²
=3(x²+xy)-2(xy+y²)
=3×5-2×(-3)
=21