x是锐角 sinx>0cosx>0 sinx+cosx>0
y=1/(2+sinx+cosx)=1/[2+√2sin(x+π/4)]
当sin(x+π/4)=1时,y有最小值ymin=1/(2+√2)=(2-√2)/2
此时x+π/4=π/2
x=π/4