By Daniel Pedoe

This revised variation of a mathematical vintage initially released in 1957 will deliver to a brand new iteration of scholars the joy of investigating that easiest of mathematical figures, the circle. the writer has supplemented this re-creation with a different bankruptcy designed to introduce readers to the vocabulary of circle innovations with which the readers of 2 generations in the past have been ordinary. Readers of Circles desire in simple terms be armed with paper, pencil, compass, and instantly area to discover nice excitement in following the buildings and theorems. those that imagine that geometry utilizing Euclidean instruments died out with the traditional Greeks can be pleasantly stunned to benefit many fascinating effects that have been merely found nowa days. beginners and specialists alike will locate a lot to enlighten them in chapters facing the illustration of a circle by way of some extent in three-space, a version for non-Euclidean geometry, and the isoperimetric estate of the circle.

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**Sample text**

Hence OX. OX'= OQ 2 This construction lies at the basis of the discussion on compass geometry in §11. We are interested in the locus of inverse points X' as X describes a given curve W. This locus is called the inverse of the curve l with respect to E. When le is a circle orthogonal to E, we have seen that the inverse is W itself. The general theorem is: The inverse of a circle is a straight line or a circle. FIa. 10 (i) The centre of inversion 0 is on A: Let OA be the diameter of W' through 0, and let A' be the inverse of A.

These joins cut the given circles in points of contact of two of the required circles. The centres of similitude of 1, '2' r lie by threes on four lines, and each line of centres of similitude gives two tangent circles. In the most favourable case we therefore expect to find eight solutions of the problem of Apollonius. We shall verify this from another point of view in chapter II. 11. Compass geometry We conclude this chapter by showing how the straight edge may be dispensed with in the elementary constructions of Euclidean geometry.

We conclude this discussion of coaxal circles by considering the radical axes, in pairs, of three circles W, '", by. 10] PROBLEM OF APOLLONIUS 21 of two of these radical axes is a point which has equal powers with respect to all three circles. This point is therefore on the third radical axis. Hence, if they are not parallel, the radical axes of the three pairs of circles which can be formed from three given circles are concurrent. The point of concurrence is called the radical centre of the three circles.