A为第四象限角,sin A = - 5/13
cosA = √(2-sin^2A) = 12/13
tanA = sinA/cosA = -5/12步骤能再详细点吗A为第四象限角，sin A＜0，cosA＞0sin A = - 5/13cosA = √(1-sin^2A) = √{1-（-5/13)^2} = 12/13tanA = sinA/cosA = (-5/13)/(12/13) =-5/12