tanA^2 = sinA^2/consA^2 = sinA^2/(1-sinA^2) = 0.2829^2/1-0.2829^2 ,开根号
所以tanA = 0.2829/根号(1-0.2829^2) 或者tanA = -0.2829/根号(1-0.2829^2)