已知x+y=3,xy=-5,求x2y+2xy2分之x2+3xy+2y2

题目

已知x+y=3,xy=-5,求x2y+2xy2分之x2+3xy+2y2

答案

x+y=3,xy=-5
（x²+3xy+2y²）/（x²y+2xy²）
=[(x+2y)(x+y)]/[xy(x+2y)]
=(x+y)/(xy)
=3/(-5)
=-3/5