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jmc

counting and probability senior

Problem

Two quadrilaterals are considered the same if one can be obtained from the other by a rotation and a translation. How many different convex cyclic quadrilaterals are there with integer sides and perimeter equal to 32?
Solution
As with solution we would like to note that given any quadrilateral we can change its angles to make a cyclic one. Let be the sides of the quadrilateral. There are ways to partition . However, some of these will not be quadrilaterals since they would have one side bigger than the sum of the other three. This occurs when . For , . There are ways to partition . Since could be any of the four sides, we have counted degenerate quadrilaterals. Similarly, there are , for other values of . Thus, there are non-degenerate partitions of by the hockey stick theorem. We then account for symmetry. If all sides are congruent (meaning the quadrilateral is a square), the quadrilateral will be counted once. If the quadrilateral is a rectangle (and not a square), it will be counted twice. In all other cases, it will be counted 4 times. Since there is square case, and rectangle cases, there are quadrilaterals counted 4 times. Thus there are total quadrilaterals.
Final answer
568