There is no simple way of factorising AB - CD
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Define abcd!
If a, b, c, and d are positive integers that add up to 20 they each must be less than 20. If abcd=81 then a, b, c, and d must be factors of 81. The factors of 81 are 1,3,9 then 27, so there is no solution to this problem.
The answer will depend on what ABCD is (or are). And since you have not bothered to provide that information, I cannot provide a sensible answer.
400 square cm
"abcd is not a parallelogram or it does not have any right angles." ~(P and Q) = ~P or ~Q