是4分之5还是5分之4?sin α是大于-1小于1的,不可能等于4分之5按5分之4算：sin (-α) = -sin α = 4/5,sin α = -4/5由α为第三象限角可知cos α为负数cos² α = 1- (4/5)²=9/25cos α = -根号下9/25 = -3/...