k=tan15°
=tan(45°-30°)
=(tan45°-tan30°)/(1+tan45°*tan30°)
=(1-√3/3)/(1+1*√3/3)
=(3-√3)/(3+√3)
=(3-√3)^2/(3+√3)(3-√3)
=(9-6√3+3)/6
=2-√3