Elements of plane and solid geometry
George Albert Wentworth
Paperback
(RareBooksClub.com, March 6, 2012)
This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1885 Excerpt: ...a regular polygon is the common centre 0 of the circumscribed and inscribed circles. 367. Def. The Radius of a regular polygon is the radius 0 A of the circumscribed circle. 368. Def. The Apothem of a regular polygon is the radius 0 s of the inscribed circle. 369. Def. The Angle at the centre is the angle included by the radii drawn to the extremities of any side. Proposition III. Theorem. 370. Each angle at the centre of a regular polygon is equal to four right angles divided, by the number of sides of the polygon. B Let ABC, etc., be a regular polygon of n sides. Ttt, it. A We are to prove Z A 0 B = Circumscribe a O about the polygon. The A A OB, BOC, etc., are equal, § 180 (in the same 0 equal arcs subtend equal A at the centre)..'. the Z A 0 B = 4 rt. A divided by the number of A about 0. But the number of A about 0 = n, the number of sides of the polygon. 4 rt. A.-.Z AOB= n Q. E. D. 371. Corollary. The radius drawn to any vertex of a regular polygon bisects the angle at that vertex. Peoposition IV. Theorem. 372. Two regular polygons of the same number of sides are similar. Let Q and Q' be two regular polygons, each having n sides. We are to prove Q and Q' similar polygons. The sum of the interior A of each polygon is equal to 2 rt. A (»--2),-§ 157 (the sum of the interior A of a polygon is equal to 1 rt. A taken as many times less 2 as the polygon has sides). 2 rt. A (n--2)., _„ Each Z of the polygon Q = ', § 158 (for the A of a regular polygon are all equal, and hence each /. is equal to the sum of the A divided by their number). Also, each /. of Q' = 2 rt-4 (n j?. § 158 n.'. the two polygons Q and Q' are mutually equiangular. A B Moreover, = lf § 363 (the sides of a regular polygon are all equal); and ±lt = 1,...