With a system of three circles it is readily seen that there are six centres of similitude, viz. two for each pair of circles, and it may be shown that these lie three by three on four lines, named the "axes of similitude." The collinear centres are the three sets of one external and two internal centres, and the three external centres.
_Coaxal Circles._--A system of circles is coaxal when the locus of points from which tangents to the circles are equal is a straight line. Consider the case of two circles, and in the first place suppose them to intersect in two real points A and B. Then by Euclid iii. 36 it is seen that the line joining the points A and B is the locus of the intersection of equal tangents, for if P be any point on AB and PC and PD the tangents to the circles, then PA·PB = PC² = PD², and therefore PC = PD. Furthermore it is seen that AB is perpendicular to the line joining the centres, and divides it in the ratio of the squares of the radii. The line AB is termed the "radical axis." A system coaxal with the two given circles is readily constructed by describing circles through the common points on the radical axis and any third point; the minimum circle of the system is obviously that which has the common chord of intersection for diameter, the maximum is the radical axis--considered as a circle of infinite radius. In the case of two non-intersecting circles it may be shown that the radical axis has the same metrical relations to the line of centres.
There are several methods of constructing the radical axis in this case. One of the simplest is: Let P and P' (fig. 5) be the points of contact of a common tangent; drop perpendiculars PL, P'L', from P and P' to OO', the line joining the centres, then the radical axis bisects LL' (at X) and is perpendicular to OO'. To prove this let AB, AB¹ be the tangents from any point on the line AX. Then by Euc. i. 47, AB² = AO² - OB² = AX² + OX² + OP²; and OX² = OD² - DX² = OP² + PD² - DX². Therefore AB² = AX² - DX² + PD². Similarly AB'² = AX² - DX² + DP'². Since PD = PD', it follows that AB = AB'.
To construct circles coaxal with the two given circles, draw the tangent, say XR, from X, the point where the radical axis intersects the line of centres, to one of the given circles, and with centre X and radius XR describe a circle. Then circles having the intersections of tangents to this circle and the line of centres for centres, and the lengths of the tangents as radii, are members of the coaxal system.