This Is a Certified Answer

Certified answers contain reliable, trustworthy information vouched for by a hand-picked team of experts. Brainly has millions of high quality answers, all of them carefully moderated by our most trusted community members, but certified answers are the finest of the finest.
See the diagram.

PQ is parallel to RS in the trapezium.    Given that A is mid point of PR and B is midpoint of QS.     Let C be midpoint of PS and  D be the midpoint of QR.

CB is line joining midpoints of two sides of the triangle PQS.  Hence, CB is parallel to PQ and also PQ = 2 * CB.

Similarly,  in triangle PRS,  CA is parallel to RS, and RS = 2 * CA
           Rs = 2 (CB + BA)  = PQ + 2 BA

         AB = 1/2 * (RS - PQ),           we assumed that RS is bigger than PQ.

If PQ is longer then RS,  then it will be  AB = 1/2 (PQ - RS)
For the  question 2 :
We don't know about the locations points L, M, P and Q,  You need to enclose a diagram.