(Sqrt(x) 即 x 的平方根）
cos a = Sqrt(1-(sin a)^2)
tan a = sin a / [Sqrt(1-(sin a)^2)]
cosec a =1 / sin a
sec a = 1 / [Sqrt(1-(sin a)^2)]
cot a = [Sqrt(1-(sin a)^2)]/ sin a