<Tr> <Td> Regular polygon </Td> <Td> 1 2 a p = 1 2 n s a (\ displaystyle (\ tfrac (1) (2)) ap = (\ tfrac (1) (2)) nsa \, \!) </Td> <Td> n (\ displaystyle n) is the number of sides, s (\ displaystyle s) is the side length, a (\ displaystyle a) is the apothem, or the radius of an inscribed circle in the polygon, and p (\ displaystyle p) is the perimeter of the polygon . </Td> </Tr> <Tr> <Td> Circle </Td> <Td> π r 2 or π d 2 4 (\ displaystyle \ pi r ^ (2) \ (\ text (or)) \ (\ frac (\ pi d ^ (2)) (4)) \, \!) </Td> <Td> r (\ displaystyle r) is the radius and d (\ displaystyle d) the diameter . </Td> </Tr> <Tr> <Td> Circular sector </Td> <Td> θ 2 r 2 or L ⋅ r 2 (\ displaystyle (\ frac (\ theta) (2)) r ^ (2) \ (\ text (or)) \ (\ frac (L \ cdot r) (2)) \, \!) </Td> <Td> r (\ displaystyle r) and θ (\ displaystyle \ theta) are the radius and angle (in radians), respectively and L (\ displaystyle L) is the length of the perimeter . </Td> </Tr> <Tr> <Td> Ellipse </Td> <Td> π a b (\ displaystyle \ pi ab \, \!) </Td> <Td> a (\ displaystyle a) and b (\ displaystyle b) are the semi-major and semi-minor axes, respectively . </Td> </Tr>

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