# IMO Training 2008 Circles Yufei Zhao. Circles. Yufei Zhao.

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1 ircles Yufei Zhao 1 Warm up problems 1. Let and be two segments, and let lines and meet at X. Let the circumcircles of X and X meet again at O. Prove that triangles O and O are similar. 2. (Miquel s theorem) Let be a triangle. Points X, Y, and Z lie on sides,, and, respectively. Prove that the circumcircles of triangles Y Z, XZ, XY meet at a common point. 3. (lso Miquel s theorem) Let a, b, c, d be four lines in space, no two parallel, no three concurrent. Let ω a denote the circumcircle of the triangle formed by lines b, c, d. Similarly define ω b, ω c, ω d. (a) Prove that ω a, ω b, ω c, ω d pass through a common point. This is called the Miquel point. (b) ontinuing the above notation, prove that the centers of ω a, ω b, ω c, ω d, and the Miquel point all lie on a common circle. 4. (Simson line) Let be a triangle, and let P be another point on its circumcircle. Let X, Y, Z be the feet of perpendiculars from P to lines,, respectively. Prove that X, Y, Z are collinear. 5. Let O be a right angle, M and N points on rays O and O, respectively. Let MNP Q be a square such that MN separates the points O and P. Find the locus of the center of the square when M and N vary. 6. Let be a convex quadrilateral such that the diagonals and are perpendicular, and let P be their intersection. Prove that the reflections of P with respect to,,, are concyclic. 7. n interior point P is chosen in the rectangle such that P + P = 180. Find P + P. 8. Let be a chord in a circle and P a point on the circle. Let Q be the projection of P on and R and S the projections of P onto the tangents to the circle at and. Prove that P Q 2 = P R P S. 9. Let be an acute triangle. The points M and N are taken on the sides and respectively. The circles with diameters N and M intersect at points P and Q. Prove that P, Q, and the orthocenter H are collinear. 10. mong the points,,, no three are collinear. The lines and intersect at, and and intersect at F. Prove that either the circles with diameters,, F pass through a common point, or no two of them have any common point. (The line through the midpoints of,, F is called the Newton-Gauss line.) 1

2 2 Tangent circles In this section, we explore the following two configurations Let chords and of a circle ω intersect at P. smaller circle ω 1 is tangent to ω at T and to segments P and P at and F respectively. P F T (a) Prove that ray T bisects arc of ω. (b) Let I be the incenter of triangle and M be the midpoint of arc of ω. Prove that M = MI = M. (c) Let F be the common point of ω 1 and line I other than. Prove that I, F,, T are concyclic. M I F' (d) Prove that F is tangent to ω 1. This means that F = F, so that, F, I are collinear. (Remember this fact.) (e) Let J be the incenter of triangle P. Prove that T,, F, J are concyclic. 1 Thanks to Oleg Golberg for providing some of these problems. T 2

3 P J F T (f) Prove that ray T J bisects angle T. 2. Now we consider a special case of the part (d) in the previous problem. Try to find a short proof of the following result. (Hint: use Pascal s theorem) Let be a triangle and I its incenter. Let Γ be the circle tangent to sides,, as well as the circumcircle of. Let Γ touch and at X and Y, respectively. Show that I is the midpoint of XY. X I Y 3. Let Ω be the circumcircle of. circle ω is tangent to sides, and circle Ω at points X, Y and Z, respectively. Let M be the midpoint of the arc of Ω which does not contain. Prove that lines XY, and ZM have a common point. (an you prove the result when Ω only passes through and and contains in the interior?) 4. circle ω is tangent to sides, of triangle and to its circumcircle at points X, Y and Z. Segments Z and XY meet at T. Prove that T X = T Y. 5. (Sawayama-Thébault) Let be a triangle with incenter I. Let a point on side. Let P be the center of the circle that touches segments,, and the circumcircle of, and let Q be the center of the circle that touches segments,, and the circumcircle of. Show that P, Q, I are collinear. 3 Projective quickies Warning: The introductory text in this section is very rough. It is not meant for you to learn from. Rather, I assume that you already know the basics of projective geometry, and this should serve as a quick review of the important concepts. You should be familiar with the notion of and. quick definition is as follows: 3

4 efinition. Suppose that ω is a circle with center O, and P and P are inverses respect to ω (i.e., P lies on ray OP such that OP OP = r 2, where r is the radius of ω). Let l be the line through P perpendicular to OP. Then we say that l is the of P and P is the of l. Here are a few diagrams that may help you to remember the common setups of s and s. Make sure that you understand the duality behind s and s. If you need to prove that something is true, it suffices to prove its dual. The map transforms points to lines, lines to points. It transforms the intersection of two lines to the line joining the two points, and vice-versa. For instance: To show that line l passes through point P, it suffices to show that the of l lies on the of P. To show that three points are collinear, it suffices to show that their s are concurrent. The cross ratio of four collinear points,,, is defined as (, ;, ) = where the lengths are directed. ross ratios are preserved under projective transformations. (esides coincidence, this is pretty much the only thing that s projective-invariant.) The most significant case is when (, ;, ) = 1, in which case we say that (, ;, ) is harmonic. This notion of cross ratios and harmonic quadruples is not limited to collinear points, but also applies to pencils of lines, and concyclic points. Harmonic quadruples arise frequently, so learn to recognize them! Possibly the most useful fact from geometry is the self-ity of the diagonal triangle of a cyclic quadrangle. (Part of the reason why it s powerful if because it is not easy to prove simply using ucliean geometry.) Theorem. Let be a cyclic quadrilateral with circumcircle ω, and let = P, = Q = R. Then P QR is self-dual with respect to ω. That is, P is the of QR, Q is the of P R, and R is the of P Q. In particular, we see that O, P, Q, R forms an orthocentric quadruple, where O is the center of ω. (This means that any of the four points is the orthocenter of the triangle formed by the other three). P Q F R 4

5 Here is a quick sketch of the proof. Let line QR meet and at and F respectively. Then from the configuration of lines, we see that (, ; P, ) and (, ; P, F ) are both harmonic. It follows the of P is F, which coincides with QR. You should develop the skill of recognizing when to use projective geometry. Here is a gallery of diagrams showing the patterns that tend to hint the use of projective geometry, in particular the use of s and s. Try to figure out the projective significance of each diagram. sk Yufei if you need help. 1. Let be a triangle and I be its incenter. Let the incenter of touch sides,, at,, F respectively. Let S denote the intersection of lines F and. Prove that SI. 2. Let UV be a diameter of a semicircle, and let P, Q are two points on the semicircle. The tangents to the semicircle at P and Q meet at R, and lines UP and V Q meet at S. Prove that RS UV. 3. circle is inscribed in quadrilateral so that it touches sides,,, at, F, G, H respectively. (a) Show that lines, F, GH are concurrent. In fact, they concur at the of. (b) Show that lines,, G, F H are concurrent. 4. (hina 1996) Let H be the orthocentre of triangle. From construct tangents P and Q to the circle with diameter, where P, Q are the points of tangency. Prove that P, H, Q are collinear. 5. (hina 1997) Let quadrilateral be inscribed in a circle. Suppose lines and intersect at P and lines and intersect at Q. From Q, construct the two tangents Q and QF to the circle where and F are the points of tangency. Prove that the three points P,, F are collinear. 6. (utterfly theorem) Let ω be a circle and let XY be a chord. Let M be the midpoint of XY, and let and be two chords of ω, both passing through M. Let XY meet chords and are P and Q respectively. So that MP = MQ. 7. Points, M, and lie on line l in that order with M = M. ircle ω is tangent to line l at. Let be the point on ω that is diametrically opposite to. Lines and meet ω at P and Q. Prove that the lines tangent to ω at P and Q and line M are concurrent. (Hint: what is the significance of midpoints in terms of harmonic conjugates?) 5

7 19. Let be a triangle with incenter I. Fix a line l tangent to the incircle of (not, or ). Let l intersect the sides of the triangle at M, N, P. t I, erect perpendiculars to IM, IN and IP and let them intersect the corresponding sides of the triangle at M 0, N 0, and P 0 respectively. Show that M 0, N 0, P 0 lie on a line tangent to the incircle. 20. Let be a triangle, and ω its incircle. Let ω touch the sides,, at,, F respectively. Let X be a point on and inside ω. Let segments X and X meet ω at Q and R respectively. Show that lines F, QR, are concurrent. Food for thought. Let be a triangle with inscribed circle ω. There exists a projective transformation that preserves ω but sends to any other desired triangle inscribed in ω! an you prove this? If a geometry problem is completely projective-invariant, then perhaps sending to an equilateral triangle or an isosceles right triangle might be a good idea. Note that when the circumcircle is not present, we can simply apply an affine transformation to get what we want. 7

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