y=2x-5-√(4x-15)
设√(4x-15)=t>=0
4x-15=t^2 x=(t^2+15)/4
y=2[(t^2+15)/4]-5-t
=1/2t^2+15/2-5-t
=1/2t^2-t+5/2
=1/2(t-1)^2+2
>=2
取最小值2时t=1 x=(t^2+15)/4=(1+15)/4=4