三角函数的特殊值 反三角函数的特殊值
arcsin0 = 0,arcsin (1/2) = π/6,arcsin(√2/2) = π/4,arcsin(√3/2) = π/3,arcsin 1 = π/2
atccos1 = 0,arccos (-1) = π,arccos 0 = π/2 .等等