cos150度=2(cos75)^2-1=-√3/2
(cos75)^2=(2-√3)/4=(8-4√3)/16
=(6-2√12-2)/16
=(√6-√2)^2/16
cos75〉0
所以cos75=(√6-√2)/4
所以arccos(√6-√2)/4=5π/12=1.3090