已知x y z 满足x+y=5,z²=xy+y-9,求x+2y+3z的值
问题描述：
已知x y z 满足x+y=5,z²=xy+y-9,求x+2y+3z的值
答
x+y=5
x=5-y
z^2=xy+y-9
z^2=(5-y)y+y-9
z^2=-y^2+6y-9
z^2=-(y-3)^2
z^2+(y-3)^2=0
所以,z=0,y-3=0
z=0,y=3
x=5-y=5-3=2
x+2y+3z=2+2*3+3*0=8