# San Jose Math Circle April 25 - May 2, 2009 ANGLE BISECTORS

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1 San Jose Math Circle April 25 - May 2, 2009 ANGLE BISECTORS Recall that the bisector of an angle is the ray that divides the angle into two congruent angles. The most important results about angle bisectors can be found below: FACT 1. The bisector of an angle consists of all points that are equidistant from the sides of the angle. In other words: Let C be a point interior to the angle AOB. Let X and Y be on OA and OB, respectively, such that CX OA and CY OB. Then: C is on the bisector of AOB if and only if CX = CY. 1

2 FACT 2. (The Angle Bisector Theorem) In any triangle, an angle bisector divides the opposite side into segments proportional to the sides of the angle. That is, if ABC is a triangle and AD is the bisector of A, then BD DC = AB AC Recall now Ceva s Theorem: Ceva s Theorem. Let M, N, and P be points on the sides BC, AC, and AB of triangle ABC, respectively. Then: AM, BN, and CP are concurrent BM MC CN NA AP P B = 1 2

3 FACT 3. In any triangle, the angle bisectors are concurrent. That is, if AD, BE, and CF are the bisectors of A, B, and C, respectively, then AD, BE, and CF are concurrent. Proof 1. Proof 2. 3

4 Let ABC be a triangle. The intersection of the angle bisectors of the triangle is usually denoted by I and is called the incenter of the triangle. Note that I is equidistant from the sides of the triangle. This common distance from the incenter to the sides is called the inradius, because the circle with center I and this radius is tangent to all three sides of the triangle. The inradius is usually denoted by r. FACT 4. If ABC is a triangle with area S, inradius r, and semiperimeter p, then r = S p Recall Stewart s Theorem, named after the eighteenth-century Scottish mathematician Matthew Stewart, although forms of the theorem were known as long ago as the fourth century A.D. Stewart s Theorem. Let A, B, and C be collinear points, in this order, and let O be a point not on the line determined by A, B, and C. Then: OA 2 BC + OC 2 AB OB 2 AC = AB BC AC 4

5 FACT 5. Let AD be the bisector of the angle BAC of the triangle ABC. Then: AD 2 = AB AC BD DC FACT 6. Let AD be the bisector of the angle BAC of the triangle ABC. Let BC = a, AC = b, and AB = c. Then BD = ac b + c and DC = ab b + c Similarly, if BE is the bisector of ABC, we have CE = ab and AE = bc a + c If CF is the bisector of BCA, then AF = bc a + b and F B = ac a + b. 5 a + c.

6 For a triangle ABC with side lengths BC = a, AC = b, AB = c, and semiperimeter p, the lengths of the angle bisectors AD, BE, and CF are usually denoted by l a, l b, and l c. Using Facts 5 and 6 we can deduce the following result: FACT 7. l a = 2 bcp(p a), lb = 2 acp(p b), and lc = 2 abp(p c). b + c a + c a + b Recall that the area S of the triangle can be calculated by the formulas: S = ab sin C 2 = bc sin A 2 = ac sin B 2 FACT 8. l a = 2bc b + c cos A 2, l b = 2ac a + c cos B 2, and l c = 2ab a + b cos C 2. 6

7 As a corollary of our previous two results we obtain formulas for the cosines of the half angles of ABC in terms of the sides: FACT 9. cos A 2 = p(p a) bc, cos B 2 = p(p b) ac, and cos C p(p c) 2 =. ab FACT 10. Let AD, BE, and CF be the angle bisectors of ABC. Let I be the incenter. Then: AI ID = b + c a, BI IE = a + c, b CI IF = a + b c 7

8 FACT 11. If I is the incenter of ABC, then bc(p a) AI =, BI = p ac(p b), CI = p ab(p c) p FACT 12. Let M, N, and P be the points of contact of the incenter with the sides of the triangle ABC, where M is on BC, N is on AC, and P is on AB. Then: AN = AP = p a, BM = BP = p b, CM = CN = p c 8

9 1. In the picture below, DP bisects ADE and EP bisects DEC. bisects ABC. Prove that BP 2. (Timisoara Mathematics Review, 1987 Middle School Annual Contest) Let ABC be a right triangle and let D be a point on the hypotenuse BC. Let DE and DF be the bisectors of the angles ADB, and ADC, respectively, where E is on AB and F is on AC. Show that: AB AE + AC AF = AD BC 9

10 3. Prove Steiner-Lehmus Theorem: Let ABC be a triangle. Then ABC is isosceles if and only if it has two congruent angle bisectors. 4. (1989 Romanian Math Olympiad, School Competition, 10th grade) Show that the triangle ABC is isosceles if and only if c(a + b) cos B 2 = b(a + c) cos C 2 10

11 5. (1989 ARML, Team Question 4) In triangle ABC, angle bisectors AD and BE intersect at P. If BC = 3, AC = 5, AB = 7, BP = x, and P E = y, compute the ratio x : y, where x and y are relatively prime integers. 6. (1992 ARML, Individual Question 18) In triangle ABC, points D and E are on AB and AC, and angle-bisector AT intersects DE at F (as shown in the diagram). If AD = 1, BD = 3, AE = 2, and EC = 4, compute the ratio AF : AT. 11

12 7. Prove that the inradius of a right triangle with legs of lengths a and b and hypotenuse c is (a + b c)/2. 8. The sides of a right triangle ABC all have integer lengths. Prove that the inradius of ABC also has integer length. 12

13 9. (1993 AIME, 15) Let CH be an altitude of ABC. Let R and S be the points where the circles inscribed in triangles ACH and BCH are tangent to CH. If AB = 1995, AC = 1994, and BC = 1993, then RS can be expressed as m/n, where m and n are relatively prime positive integers. Find m + n. 13

14 10. (2008 AMC 12A, 20) Triangle ABC has AC = 3, BC = 4, and AB = 5. Point D is on AB and CD bisects the right angle. The inscribed circles of ADC and BCD have radii r a and r b, respectively. What is r a r b? (A) 1 28 (10 2) (B) 3 56 (10 2) (C) 1 14 (10 2) (D) 5 56 (10 2) (E) 3 28 (10 2) 11. (1997 ARML, Team Contest 6) In ABC, D lies on AC so that ABD = DBC = θ. If AB = 4, BC = 16, and BD = 2 cos θ, then BD = a b for relatively prime integers a and b. Compute the ordered pair (a, b). 14

15 12. (2001 AIME I, 4) In triangle ABC, angles A and B measure 60 degrees and 45 degrees, respectively. The bisector of angle A intersects BC at T, and AT = 24. The area of the triangle ABC can be written in the form a + b c, where a, b, and c are positive integers, and c is not divisible by the square of any prime. Find a + b + c. 13. (2001 AIME I, 7) Triangle ABC has AB = 21, AC = 22, and BC = 20. Points D and E are located on AB and AC, respectively, such that DE is parallel to BC and contains the center of the inscribed circle of triangle ABC. Then DE = m/n, where m and n are relatively prime positive integers. Find m + n. 15

16 14. (2001 AIME II, 7) Let P QR be a right triangle with P Q = 90, P R = 120, and QR = 150. Let C 1 be the inscribed circle. Construct ST, with S on P R and T on QR such that ST is perpendicular to QR and tangent to C 1. Construct UV with U on P Q and V on QR such that UV is perpendicular to P Q and tangent to C 1. Let C 2 be the inscribed circle of RST and C 3 the inscribed circle of QUV. The distance between the centers of C 2 and C 3 can be written as 10n. What is n? 16

17 15. In ABC, AB = AC, BAC = 36, and BC = 1. Find AB. 16. Let ABCDE be a regular pentagon inscribed in a unit circle. Prove that the product of the lengths of the two sides and the diagonals from a vertex is 5, that is, in the figure below, prove that AB AC AD AE = 5 17

18 17. (Timisoara Mathematics Review, 1987 Middle School Annual Contest) Show that there does not exist a triangle whose incircle divides an angle bisector into three congruent segments. 18. In triangle ABC, rays BD and BE trisect ABC. If DE = 4, EC = 5, and BD = 12, find BC, BE, and AD. 18

19 19. Is it possible for the angle trisectors of angle A of triangle ABC to divide side BC into three congruent segments? Justify your answer. 20. (1981 AMC 12, 25) In triangle ABC in the adjoining figure, AD and AE trisect BAC. The lengths of BD, DE, and EC are 2, 3, and 6, respectively. The length of the shortest side of ABC is: (A) 2 10 (B) 11 (C) 6 6 (D) 6 (E) not uniquely determined by the given information 19

20 21. Let l 1 and l 2 be the trisectors of BAC of triangle ABC. Show that: ( 1 l 1 c + 1 ) ( 1 = l 2 l 2 b + 1 ) l Let AD and AE be the trisectors of BAC of triangle ABC, where D and E are on BC. Show that: A 3 1 AD 1 2R sin 2 AE = 2 BC ( 1 AB 1 ) 2 AC 2 20

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