设 sinθ+cosθ＝√2sin(x+π/4)＝t
则：t属于[-√2,√2]
sinxcosx=(t^2-1)/2
y=t^2/2+t-1/2=1/2(t+1)^2-1
最大值是：√2+1/2(此时t=√2)