（ 根号27 - 1/(2-根号3)）*根号12
={3根号3 - (2+根号3)/[(2+根号3)(2-根号3)]}*2根号3
=[3根号3 - (2+根号3)]*2根号3
=(2根号3 -2)*2根号3
=12 - 4根号3