向量............

Something wrong with the question. Should that be AD // BC and to prove

→-----→-----→----→-------→-----→
AD + AB + CB + CD = 2(CB + AD)

If so,
------------→-----→-----→----→
L.H.S. = AD + AB + CB + CD

----→----→-------→-------→------→------→
= ( AC + CD ) + ( AC + CB ) + CB + CD

----→-------→------→
= 2AC + 2CD + 2CB

----→-----=→
= 2AD + 2CB

------→------→
= 2 ( CB + AD )

= R.H.S.




2007-07-14 15:36:33 補充：
箭咀位置有點對不正, 請見諒。