cos(15°-α)=sin(90°-(15°-α))=sin(75°+α)=-平方根(1-cos(75°+α)的平方)=-2平方根（2）/3
cos(105°-α)=cos(180-(75°+α))=-cos(75°+α)=-1/3
sin(α-105度)=-sin(180+α-105)=-sin(75+α)=+2平方根（2）/3