如图，顺次连接圆内接矩形各边的中点，得到菱形ABCD，若BD＝8，DF＝4，则菱形ABCD的边长为多少？

    passage 1

    Articles Wanted

    School Life

    Articles should be typewritten and double spaced, using only one side of the page.

    Your name, year of birth, school name (and English teacher), home address and e-mail address must be included.

    For photos, place the information on the back of each envelope: PLEASE DON'T FOLD.

    Please keep copies of your text, pictures or photos.

    The sentence MUST BE WRITTEN with your name on each work, "I promise the above work is completely original (原创的)."

    If your article is chosen, you will receive a copy of School Life and a special gift.

    SEND IT ALL SUMMER!

    For more information, please contact (联系) us!

    Tel: (0417) 381361 (0417) 386743

    E-mail: schoollife@acpmagazines.com.

    Mail: School Life. Box 5252, Yingkou, NSW 200

人乘电梯匀速上升，在此过程中人受到的重力为G，电梯对人的支持力为F_N，人对电梯的压力为F_N’，则（   ）

下图为“美国和我国东北地区农业分布示意图”。读图回答下列问题。