∵向量OA=（3,-√3）向量OB=（cosa,sina）∴OA●OB=3cosa-√3sina|A\OA|=√(3²+3)=2√3,|OB|=√(cos²a+sin²a)=1∴cos=OA●OB/(|OA||OB|) =(3cosa-√3sina)/(2√3) =√3/2cosa-1/2*sina=cos(a...