设 sinθ+cosθ＝√2sin(x+π/4)＝t则：t属于[-√2, √2]sinxcosx=(t^2-1)/2y=t^2/2+t-1/2=1/2(t+1)^2-1最大值是：√2+1/2(此时t=√2)
y=sinθ+cosθ+sinθcosθ =根号2*sin（θ+π/4）+（sin2θ）/2当θ=π/4时，y将取得最大值因为这时sin（θ+π/4）=sin2θ=1故y的最大值是：根号2+1/2
