x0
y=(4x-5)+1/(4x-5)+3
=-[(5-4x)+1/(5-4x)]+3
(5-4x)+1/(5-4x)≥2√[(5-4x)*1/(5-4x)]=2
所以-[(5-4x)+1/(5-4x)]≤-2
y≤-2+3=1
最大值是1