3tan105°/(1-tan^2 75°）
=3tan(180-75)/(1-(tan75)^2)
=-3/2*2tan75/[1-(tan75)^2]
=-3/2*tan(75*2)
=-3/2*tan(180-30)
=-3/2*(-tan30)
=3/2*根号3/3
=根号3 /2