Connect eg with the midpoint of AD as g; ?

EG= 1/2CD=2

Similarly, take the midpoint h of BC and connect FH.

FH= 1/2CD=2

GH=(AB+CD)/2？ =? seven

So EF? =? GH-EG-FH？ =? 3?

I proved it, you can also Baidu.

The center line of your lower trapezoid must pass through the midpoint of its diagonal; Many answers

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