Question

已知实数x,y,z满足x-y=5,z^2=-xy-y-9,求x-2y+3z的值

Answer 1

x-y=5

x=5+y

z^2=-xy-y-9=-(5+y)y-y-9=-y^2-6y-9=-(y+3)^2

所以,z=0,y+3=0

z=0,y=-3

x=5+y=5-3=2

x-2y+3z=2-2*(-3)+0=8