椭圆kx^2+(k+2)y^2=k的焦点在y轴上,
kx^2+(k+2)y^2=k
x^2+y^2/[k/(k+2)]=1
焦点在y轴上,
k/(k+2)>1
k/(k+2)-1=-2/(k+2)>0
k+2<0
k<-2