Isosceles trapezoid

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An isosceles trapezoid is a convex quadrilateral with one pair of parallel sides, called the bases, and the other two sides, called the legs, are equal in length. It has a line of symmetry that passes through the midpoints of the bases, which makes the base angles equal. The diagonals are also equal in length.

Key properties:
- The two base angles on each base are equal.
- Angles next to opposite bases are supplementary.
- The diagonals have the same length.
- The height is the perpendicular distance between the bases.

Area formula:
- If the bases have lengths a and b and the height is h, the area is K = (a + b)/2 × h.
- If you know the leg length c and the bases a and b, you can find the height: h = sqrt(c^2 − ((a − b)/2)^2). Then K = (a + b)/2 × sqrt(c^2 − ((a − b)/2)^2).

Special cases:
- Rectangles and squares are usually considered special isosceles trapezoids.
- Parallelograms are not isosceles trapezoids (unless it happens to be a rectangle).