2、带长L L=AB+bc+cd+ da D a X (7-20) 2 =—兀-c 2 D BC=a,x2=(丌+20)× 2 C 兀-c 6 2 AB=CD=acos e ∴cos日=√1-sin26≈1--62+-04--0 取:cos=√1-sin2b L≈2a+(d1+d2)+-2-d)2 6 2 4a 2a2、带长L L = AB+ BC+ CD + DA 2 2 2 2 2 1 1 1 1 1 d d d d DA = − = = − ( ) 2 2 2 2 2 2 2 2 2 2 d d d d BC = − = = + ( ) AB = CD = acos = − 2 − 2 + 4 − 6 + 6! 1 4! 1 2! 1 cos 1 sin 1 a d d L a d d 2 4 2 2 2 1 1 2 ( ) ( ) − + + + a d d 2 cos 1 sin 2 1 2 − = − 取: