f (x )=4x +9/(x -5)=4(x-5)+9/(x-5)+20>=2√[4(x-5)*9/(x-5)]+20 x>5 则x-5>0
=2*√36+20=12+20=32
所以最小值是32
求f (x )=4x +9/(x -5)(x >5)的最小值.
f (x )=4x +9/(x -5)=4(x-5)+9/(x-5)+20>=2√[4(x-5)*9/(x-5)]+20 x>5 则x-5>0
=2*√36+20=12+20=32
所以最小值是32