sinA+cosA=根号2*sin(A+π/4)因为 0＜A＜π/2所以 π/4＜A+π/4＜3π/4所以 根号2/2＜sin(A+π/4)＜1所以 sinA+cosA=根号2*sin(A+π/4)＞1
