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