Question

已知x,y,z满足x+y=5,z^2=xy+y-9 求x+2y+3z的值

Answer 1

x=5-y
z2=(5-y)y+y-9
=6y-y2-9
=-(9-6y+y2)
=-(y-3)2
由题意，只有当该项为0时等式成立
得y=3
那么z=0
x=2
即原式=2+6+0=8。请采纳回答

Answer 2

x=5-y
z2=(5-y)y+y-9
=6y-y2-9
=-(9-6y+y2)
=-(y-3)2
由题意，只有当该项为0时等式成立
得y=3
那么z=0
x=2
即原式=2+6+0=8

Answer 3

x=5-y,因为z^2=xy+y-9=(5-y)y+y-9=6y-y^2-9=-(y-3)^2,
又因为z^2>=0,所以y=3,x=2.z^2=2*3+3-9=0.所以x+2y+3z=2+2*3+3*0=8