It is known that it is possible to inscribe a circle into a quadrilateral only if the sums of its opposite sides are equal.
We have AB and DC – opposite sides.
2CD = AB;
CD = 0.5AB;
AB + DC = AB + 0.5AB = 1.5AB.
The sum of the other two sides must also equal 1.5AB.
Perimeter p = 1.5AB + 1.5AB = 3 * AB = 3 * 5 = 15.
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