已知实数x.y.z满足x+y=5及z*z=xy+y-9则x+2y+3z=?

x+y=5
x=5-y
z^2=(5-y)y+y-9
=6y-y^2-9
=-(9-6y+y^2)
=-(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
x+2y+3z=2+2*3+3*0=2+6+0=8

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