a=(sin15,cos15),b=(cos15,sin15)a·b=(sin15,cos15)·(cos15,sin15)=sin15cos15+cos15sin15=sin30=1/2|a-b|^2=|a|^2+|b|^2-2a·b=2-1=1即：|a-b|=1a·(a-b)=|a|^2-a·b=1-1/2=1/2故：cos=a·(a-b)/(|a|*|a-b|)=1/2...