sin(π/12）=sin（π/4-π/6）=sin（π/4）cos（π/6）-cos（π/4）sin（π/6）=（根号6-根号2）/4
cos(π/12）=cos（π/4）cos（π/6）+sin（π/4）sin（π/6）=（根号6+根号2）/4
∴tan(π/12）=sin(π/12）/cos(π/12）=2-根号3
∴f（π/12）=-4倍根号3