数字平行四边形知识

已知：平行四边形ABCD，O是对角线的交点，EF过的O，且EF⊥AD

求证：OE=OF

       (借用yezi6672的图，谢谢)

证明：∵四边形ABCD是平行四边形

∴AD∥BC，OB=OD

∴∠OBF=∠ODE

∵EF⊥AD

∴EF⊥BC

∴∠OFB=∠OED=90º

∴ΔOFB≌ΔOED

∴OE=OF

1.平行四边形对边平行、对边长度相等，被两条对角线分成的四个三角形中，对顶的两个全等（角角边）
2.全等的两个三角形的高相等（就是交点到对边的距离）

作OE⊥AD, OF⊥BC

∵四边形ABCD是平行四边形

∴AD∥BC ,OA=OC ,∠DAC=∠BCA ,∠AEO=∠CFO

∴△AOE≌△COF
∴OE=OF

1-10 one two three four five six seven eight nine ten
11-20 eleven twelve thirteen fourteen fifteen sixteen seventeen eighteen nineteen twenty
21-29 twenty-one twenty-two twenty-three twenty-four....
30-39 thirty thirty-one thirty-two ....
40-49 forty forty-one forty-two forty-three....
50-59 fifty fifty-one fifty-two fifty-three.....
60-69 sixty sixty-one sixty-two sixty-three.....
70-79 seventy seventy-one seventy-two .....
80-89 eighty eighty-one eighty-two eighty-three....
90-99 ninety ninety-one ninety-two ninety-three...
100 one hundred

一月至十二月:
January - February - March - April - May - June - July - August - September - October - November - December

星期：
Monday 星期一 Tuesday 星期二 Wednesday 星期三 Thursday 星期四 Friday 星期五 Saturday 星期六 Sunday 星期天

时间:
second 秒, minute分 , quarter一刻钟 ,hour 小时 ,day 天 ,week 周 ,month 月 ,year 年
century 世纪
morning早晨 ,noon中午 ,afternoon 下午 night傍晚 ,midnight深夜