17. With three given points as centres describe three circles touching one another two by two. How many solutions will there be ? 18. Two tangents AB, AC are drawn to a given circle from an external point A; and in AB, AC two points D and E are taken so that DE is equal to the sum of DB and EC: shew that DE touches the circle. 19. Given the perimeter of a triangle, and one angle in magnitude and position : shew that the opposite side always touches a fixed circle. 20. Given the centres of the three escribed circles ; construct the triangle. 21. Given the centre of the inscribed circle, and the centres of two escribed circles ; construct the triangle. 22. Given the vertical angle, perimeter, and the length of the bisector of the vertical angle ; construct the triangle. 23. Given the vertical angle, perimeter, and altitude ; construct the triangle. 24. Given the vertical angle, perimeter, and radius of the inscribed circle ; construct the triangle. 25. Given the vertical angle, the radius of the inscribed circle, and the length of the perpendicular from the vertex to the base ; construct the triangle. 26. Given the base, the difference of the sides containing the vertical angle, and the radius of the inscribed circle ; construct the triangle. [See Ex. 10, p. 276.] 27. Given a vertex, the centre of the circumscribed circle, and the centre of the inscribed circle, construct the triangle. 28. In a triangle ABC, I is the centre of the inscribed circle ; shew that the centres of the circles circumscribed about the triangles BIC, CIA, AIB lie on the circumference of the circle circumscribed about the given triangle. 29. In a triangle ABC, the inscribed circle touches the base BC at D; and r, r, are the radii of the inscribed circle and of the escribed circle which touches BC: shew that r. 1.91=BD.DC. 30. ABC is a triangle, D, E, F the points of contact of its inscribed circle ; and D'E'F' is the pedal triangle of the triangle DEF : shew that the sides of the triangle D'E'F' are parallel to those of ABC. 31. In a triangle ABC the inscribed circle touches BC at D. Shew that the circles inscribed in the triangles ABD, ACD touch one another. ON THE NINE-POINTS CIRCLE. 32. In any triangle the middle points of the sides, the feet of the perpendiculars drawn from the vertices to the opposite sides, and the middle points of the lines joining the orthocentre to the vertices are concyclic. In the A ABC, let X, Y, Z be the middle points of the sides BC, CA, AB; let D, E, F be the feet of the perps drawn to these sides fronı A, B, C; let O be the orthocentre, and a, B, y the middle points of OA, OB, OC. Then shall the nine points X, Y, Z, D, E, F, a, b, y be concyclic. Join XY, XZ, Xa, Ya, Za. Now from the A ABO, B Х since AZ=ZB, and Aa=aO, Hyp. i. Za is par to BO. Ex. 2, p. 104. And from the A ABC, since BZ=ZA, and BX=XC, :: ZX is parl to AC. But BO produced makes a rt. angle with AC ; Hyp. :: the L XZa is a rt. angle. Similarly, the L XYa is a rt. angle. : the points X, Z, a, Y are concyclic: that is, a lies on the Oce of the circle, which passes through X, Y, Z; and Xa is a diameter of this circle. Similarly it may be shewn that ß and y lie on the Oce of the circle which passes through X, Y, Z. Again, since a DX is a rt. angle, Hyp. : the circle on Xa as diameter passes through D. Similarly it may be shewn that E and F lie on the circumference of the same circle. .. the points X, Y, Z, D, E, F, a, ß, y are concyclic. Q.E.D. Нур. I. 29. a a From this property the circle which passes through the middle points of the sides of a triangle is called the Nine-Points Circle ; many of its properties may be derived from the fact of its being the circle circumscribed about the pedal triangle. 33 To prove that (i) the centre of the nine-points circle is the middle point of the straight line which joins the orthocentre to the circumscribed centre. (ii) the radius of the nine-points circle is half the radius of the circumscribed circle. (iii) the centroid is collinear with the circumscribed centre, the nine-points centre, and the orthocentre. In the A ABC, let X, Y, Z be the middle points of the sides ; D, E, F the feet of the perps ; O the orthocentre; S and N the centres of the circumscribed and nine-points circles respectively. (i) To prove that N is the middle point of so. E It may be shewn that the perp. to XD from its middle point bisects Ex. 14, p. 106. Similarly the the perp. to EY at its X D с middle point bisects SO: that is, these perps intersect at the middle point of SO: And since XD and EY are chords of the nine-points circle, :: the intersection of the lines which bisect XD and EY at rt. angles is its centre : III. 1. :. the centre N is the middle point of SO Y SO; a (ii) To prove that the radius of the nine-points circle is half the radius of the circumscribed circle. By the last Proposition, Xa is a diameter of the nine-points circle. :. the middle point of Xa is its centre: but the middle point of SO is also the centre of the nine-points circle. (Proved.) SN=ON, and NX=Na, I. 15. :: SX=Oa I. 4. =Aa. And SX is also parl to Aa, :: SA=Xa. I. 33. But SA is a radius of the circumscribed circle ; and Xa is a diameter of the nine-points circle ; .. the radius of the nine-points circle is half the radius of the cir cumscribed circle. Ex. 1, (iii) To prove that the centroid is collinear with points S, N, O. Join AX and draw ag parl to so. Let AX meet SO at G. Then from the AGO, since Aa=aO, and ag is parł to OG, .. Ag=gG. p. 104. And from the Xag, since aN=NX, and NG is parł to ag, :: gG=GX. :: AG=ş of AX ; :: G is the centroid of the triangle ABC. Ex. 4, p. 113. That is, the centroid is collinear with the points S, N, O. Q. E. D. 34. Given the base and vertical angle of a triangle, find the locus of the centre of the nine-points circle. 35. The nine-points circle of any triangle ABC, whose orthocentre is O, is also the nine-points circle of each of the triangles AOB, BOC, COA. 36. If I, 11, 12, 13 are the centres of the inscribed and escribed circles of a triangle ABC, then the circle circumscribed about ABC is the nine-points circle of each of the four triangles formed by joining three of the points I, 11, 12, 13. 37. All triangles which have the same orthocentre and the same circumscribed circle, have also the same nine-points circle. 38. Given the base and vertical angle of a triangle, shew that one angle and one side of the pedal triangle are constant. 39. Given the base and vertical angle of a triangle, find the locus of the centre of the circle which passes through the three escribed centres. Note. For another important property of the Nine-points Circle see Miscellaneous Examples on Book VI., Ex. 60. 1. If four circles are described to touch every three sides of a quadrilateral, shew that their centres are concyclic. 2. If the straight lines which bisect the angles of a rectilineal figure are concurrent, a circle may be inscribed in the figure. 3. Within a given circle describe three equal circles touching one another and the given circle. 4. The perpendiculars drawn from the centres of the three escribed circles of a triangle to the sides which they touch, are concurrent. 5. Given an angle and the radii of the inscribed and circum. scribed circles ; construct the triangle. 6. Given the base, an angle at the base, and the distance between the centre of the inscribed circle and the centre of the escribed circle which touches the base ; construct the triangle. 7. In a given circle inscribe a triangle such that two of its sides may pass through two given points, and the third side be of given length. 8. In any triangle ABC, I, 11, 12, 13 are the centres of the inscribed and escribed circles, and S, S, S, are the centres of the circles circumscribed about the triangles BỨC, CIA, AIB : shew that the triangle S,S,S, has its sides parallel to those of the triangle 111213, and is one-fourth of it in area : also that the triangles ABC and S,S,S, have the same circumscribed circle. 9. O is the orthocentre of a triangle ABC: shew that AO2 + BC?=BO2 + CAP=CO2 + AB2=d?, where d is the diameter of the circumscribed circle. 10. If from any point within a regular polygon of n sides perpendiculars are drawn to the sides, the sum of the perpendiculars is equal to n times the radius of the inscribed circle. 11. The sum of the perpendiculars drawn from the vertices of a regular polygon of n sides on any straight line is equal to n times the perpendicular drawn from the centre of the inscribed circle. 12. The area of a cyclic quadrilateral is independent of the order in which the sides are placed in the circle. 13. Given the orthocentre, the centre of the nine-points circle, and the middle point of the base; construct the triangle. 14. Of all polygons of a given number of sides, which may be inscribed in a given circle, that which is regular has the maximum area and the maximum perimeter. 15. Of all polygons of a given number of sides circumscribed about a given circle, that which is regular has the minimum area and the niinimum perimeter. 16. Given the vertical angle of a triangle in position and magniturle, and the sum of the sides containing it: find the locus of the centre of the circumscribed circle. 17. P is any point on the circumference of a circle circumscribed about an equilateral triangle ABC: shew that PA2+PB2+ PCP is constant. |