(1-tan^2π/12)/(tanπ/12)
=2*(1-tan^2π/12)/(2tanπ/12)
=2/(tanπ/6)
=2√3/3=2*(1-tan^2π/12)/(2tanπ/12) =2/(tanπ/6) 这里我没办法理解啊tan2a=2tana/(1-tan^2a)就是这个公式取倒数