cosx-sinx=t (cosx-sinx)^2=1-2sinxcosx=t^2
sinxcosx=1-t^2/2
y=1-cosx+sinx+sinxcosx=1-t+(1-t^2)/2=-t^2/2-t+3/2=-1/2[t^2+2t+1]+2
对称轴t=-1
在【1,√2】上单调减
值域[1/2-√2,0]