设圆半径为r,延长OA至D,使OD=3r,延长OB至E,使OE=4r,延长CO至F,使OF=√13r,3OA+4OB+√13OC=0(全为向量),表示ODFE为平行四边形由余弦定理cos(∠DOF)=(DO^2+OF^2-FD^2)/(2*OD*OF)=1/√13cos(∠FOE)=2.5/√13由和角公式s...