Question

x-2y=15,xy=-25求x和y的值

Answer 1

x-2y=15得x=2y+15

xy=-25得（2y+15)y=-25

2y^2+15y+25=0

(2y+5)(y+5)=0

因此2y+5=0或y+5=0

y=-2.5或y=-5

y=-2.5时,x=2X(-2.5)+15=10

y=-5时,x=2X(-5)+15=5