證明:(1)

四邊形

是平行四邊形,

.

.················································································· 2分

,

.

.·················································································· 3分

········································································ 4分
(2)由

得

.

,·········································································· 5分

四邊形

是平行四邊形. ·································································· 7分


. 8分解析:
略