t=sinx-cosx=√2sin(x-π/4)
0≤x≤π
-π/4≤x-π/4≤3π/4
√2sin(-π/4)≤t≤√2sin(π/2)
-1≤t≤√2
y=1/2（1-t^2）+t=-1/2(t-1)^2+1
-1≤t≤√2
-1≤y≤1