Question

已知x,y∈R+,且9x+y-xy=0,则x+y的最小值为

Answer 1

9x+y-xy=0

x=y/(y-9)

x+y=y/(y-9)+y

=(y-9+9)/(y-9)+y

=1+9/(y-9)+y-9+9

=10+9/(y-9)+(y-9) a+1/a>=2

>=10+2*3

当y=6时有 最小值是16