y=2x－5＋√15－4x
=1/2(4x-15)+2.5＋√15－4x
设z=√15－4x
则z>=0
y=-1/2z^2 +z+2.5
=-1/2(z-1)^2+3
所以z=1即=√15－4x=1时,y有最大值3
所以y