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The area of polygon with given sides is obviously largest when the polygon is convex.
Given a convex quadrilateral $ABCD$, we can interchange the length of two consecutive sides without changing the area. Indeed, we can flip the triangle $ABC$ with respect to the perpendicular bisector of $AC$ to get a quadrilateral $AB'CD$ with the same area and perimeter. As transpositions generate the whole symmetric group $S_4$, we may permute the lengths of the sides in any way without changing the solution. Note that this is true for any convex polygon.
In this case, we may therefore assume that equal sides are not adjacent; the quadrilateral is therefore a parallelogram with sides $a$ and $b$. The area will be maximum and equal to $ab$ when that parallelogram is a rectangle.
The original quadrilateral will be either a rectangle or a kite-shaped figure with two opposite right angles (a right triangle and its mirror image with respect to the hypotenuse).
Note: it can be shown that, for a quadrilateral with any given sides, the area is largest when the quadrilateral is cyclic; and that too can be generalized to any polygon. See
here for a proof.
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