cos(π/2+a)=1/3
-sina = 1/3
sina = -1/3
tan(3π/2-a)
= tan(π/2-a)
= cota
= cosa/sina
= ±2√2/3cosa怎么求?考虑直角三角形，对边= 1，斜边=3， 邻边= (3^2-1^2) = 2√2不过答案貌似是错的