2sin^2x+cos^2y=1
cos^2y=1-2sin^2x≥0
∴0≤1-2sin^2x≤1
∴0≤sin^2x≤1/2
∴sin^2x+cos^2y=sin^2x+1-2sin^2x=1-sin^2x
∵0≤sin^2x≤1/2
∴1/2≤1-sin^2x≤1
∴1/2≤sin^2x+cos^2y≤1-1≤1-2sin^2x=cos2x ≤1哪里不对？1-sin^2x 跟1-2sin^2x不一样多了个2