v(a,6) (1+cos)-2-(1+cosa)+ +a-7 4丌 4丌 4丌(1 4丌 +c)2(+a2+(+++ (1+ cos B)l 4x4丌a( ) ( ) ( ) ( ) 2 * ( , ) 1 cos 1 cos 4 4 4 * * 1 cos 1 cos (1 cos ) (1 cos ) 4 4 4 Q Q q a a a l Q Q Q l a                  = − + − + + + −       = − + − + + + + +           = + − + a Q Q l    4 * 4 (1 cos )