y=sinx+cosx=√2sin(x+π/4)
y'=√2cos(x+π/4)当-π/2≤x≤0时,-π/4≤x+π/4≤π/4,此时：y'＞0,所以函数是增函数.
所以：min{y}=√2sin(-π/4)=-1,max{y}=√2sin(π/4)=1.