设x²＋xy=3,xy＋y²＝2,求2x²－xy－3y²的值
问题描述：
设x²＋xy=3,xy＋y²＝2,求2x²－xy－3y²的值
答
2x²-xy-3y²
=2(x²+xy)-3xy-3y²
=2(x²+xy)-3(xy+y²)
=2×3-3×2
=0
答
2x²－xy－3y²
=2x²+2xy－3y²-3xy
=2(x²+xy)－3(y²+xy)
=2*3-3*2
=0
答
2x²－xy－3y²
=2x²+2xy-2xy－xy－3y²
=2x²+2xy-3xy－3y²
=2(x²+xy)-3(xy+y²)
x²＋xy=3,xy＋y²＝2
带入
2*3-3*2=0