་

61. Two circles intersect at A and B, and through A two straight lines HAK, PAQ are drawn terminated by the circumferences: if HP and KQ intersect at X, shew that the points H, B, K, X are concyclic.

62. Describe a circle touching a given straight line at a given point, so that tangents drawn to it from two fixed points in the given line may be parallel. [See Ex. 10, p. 183.]

63. C is the centre of a circle, and CA, CB two fixed radii: if from any point P on the arc AB perpendiculars PX, PY are drawn to CA and CB, shew that the distance XY is constant.

64. AB is a chord of a circle, and P any point in its circumference; PM is drawn perpendicular to AB, and ÂN is drawn perpendicular to the tangent at P: shew that MN is parallel to PB.

65. P is any point on the circumference of a circle of which AB is a fixed diameter, and PN is drawn perpendicular to AB; on AN and BN as diameters circles are described, which are cut by AP, BP at X and Y shew that XY is a common tangent to these circles.

66. Upon the same chord and on the same side of it three segments of circles are described containing respectively a given angle, its supplement and a right angle: shew that the intercept made by the two former segments upon any straight line drawn through an extremity of the given chord is bisected by the latter segment.

67. Two straight lines of indefinite length touch a given circle, and any chord is drawn so as to be bisected by the chord of contact: if the former chord is produced, shew that the intercepts between the circumference and the tangents are equal.

68. Two circles intersect one another: through one of the points of contact draw a straight line of given length terminated by the circumferences.

69. On the three sides of any triangle equilateral triangles are described remote from the given triangle: shew that the circles described about them intersect at a point.

70. On BC, CA, AB the sides of a triangle ABC, any points P, Q, R are taken; shew that the circles described about the triangles AQR, BRP, CPQ meet in a point.

71. Find a point within a triangle at which the sides subtend equal angles.

72. Describe an equilateral triangle so that its sides may pass through three given points.

73. Describe a triangle equal in all respects to a given triangle, and having its sides passing through three given points.

SIMSON'S LINE.

74. If from any point on the circumference of the circle circumscribed about a triangle, perpendiculars are drawn to the three sides, the feet of these perpendiculars are collinear.

Let P be any point on the Oce of the circle circumscribed about the ▲ ABC; and let PD, PE, PF be the perps drawn from P to the three sides.

It is required to prove that the points D, E, F are collinear.

E

C

Hyp.

Because the PDB, PFB are rt. angles, .. the points P, D, B, F are concylic: ..the PDF the PBF, in the same segment. But since BACP is a quad' inscribed in a circle, having one of its

sides AB produced to F, .. the ext.

.. the

then the

.. the

PBF

III. 21.

the opp. int. ACP. Ex. 3, p. 188. PDF the ACP.

To each add the PDE:

PDF, PDE = the ECP, PDE.
But since the 2 PDC, PEC are rt. angles,
.. the points P, D, E, C are concylic;
ECP, PDE together two rt. angles:
PDF, PDE together two rt. angles;
.. FD and DE are in the same st. line;
that is, the points D, E, F are collinear.

.. the

1. 14.

Q. E.D.

[The line FDE is called the Pedal or Simson's Line of the triangle ABC for the point P; though the tradition attributing the theorem to Robert Simson has been recently shaken by the researches of Dr. J. S. Mackay.]

75. ABC is a triangle inscribed in a circle; and from any point P on the circumference PD, PF are drawn perpendicular to BC and AB: if FD, or FD produced, cuts AC at E, shew that PE is perpendicular to AC.

76. Find the locus of a point which moves so that if perpendicu lars are drawn from it to the sides of a given triangle, their feet are collinear.

77. ABC and AB'C' are two triangles having a common vertical angle, and the circles circumscribed about them meet again at P: shew that the feet of perpendiculars drawn from P to the four lines AB, AC, BC, B'C' are collinear.

78. A triangle is inscribed in a circle, and any point P on the circumference is joined to the orthocentre of the triangle: shew that this joining line is bisected by the pedal of the point P.

IV.

ON THE CIRCLE IN CONNECTION WITH RECTANGLES.

See Propositions 35, 36, 37.

1. If from any external. point P two tangents are drawn to a given circle whose centre is O, and if OP meets the chord of contact at Q; then the rectangle OP, OQ is equal to the square on the radius.

Let PH, PK be tangents, drawn from

the external point P to the & HAK, whose centre is O; and let OP meet HK the chord of contact at Q, and the Oce at A: then shall the rect. OP, OQ=the sq. on OA.

On HP as diameter describe a circle : this circle must pass through Q, since the HQP is a rt. angle.

Join OH.

H

A

III. 31.

K

III. 16.

III. 36.

Q. E. D.

2. ABC is a triangle, and AD, BE, CF the perpendiculars drawn from the vertices to the opposite sides, meeting in the orthocentre O: shew that the rect. AO, OD= the rect. BO, OE=the rect. CO, OF.

3. ABC is a triangle, and AD, BE the perpendiculars drawn from A and B on the opposite sides: shew that the rectangle CA, CE is equal to the rectangle CB, CD.

4. ABC is a triangle right-angled at C, and from D, any point in the hypotenuse AB, a straight line DE is drawn perpendicular to AB and meeting BC at E: shew that the square on DE is equal to the difference of the rectangles AD, DB and CE, EB.

5. From an external point P two tangents are drawn to a given circle whose centre is O, and OP meets the chord of contact at Q: shew that any circle which passes through the points P, Q will cut the given circle orthogonally. [See Def. p. 222.]

6. A series of circles pass through two given points, and from a fixed point in the common chord produced tangents are drawn to all the circles: shew that the points of contact lie on a circle which cuts all the given circles orthogonally.

7. All circles which pass through a fixed point, and cut a given circle orthogonally, pass also through a second fixed point.

8. Find the locus of the centres of all circles which pass through a given point and cut a given circle orthogonally.

9. Describe a circle to pass through two given points and cut a given circle orthogonally.

10. A, B, C, D are four points taken in order on a given straight line: find a point O between B and C such that the rectangle OA, OB may be equal to the rectangle OC, OD.

11. AB is a fixed diameter of a circle, and CD a fixed straight line of indefinite length cutting AB or AB produced at right angles; any straight line is drawn through A to cut CD at P and the circle at Q: shew that the rectangle AP, AQ is constant.

12. AB is a fixed diameter of a circle, and CD a fixed chord at right angles to AB; any straight line is drawn through A to cut CD at P and the circle at Q: shew that the rectangle AP, AQ is equal to the square on AC.

13. A is a fixed point and CD a fixed straight line of indefinite length; AP is any straight line drawn through A to meet CD at P; and in AP a point Q is taken such that the rectangle AP, AQ is constant: find the locus of Q.

14. Two circles intersect orthogonally, and tangents are drawn from any point on the circumference of one to touch the other prove that the first circle passes through the middle point of the chord of contact of the tangents. [Ex. 1, p. 233.]

15. A semicircle is described on AB as diameter, and any two chords AC, BD are drawn intersecting at P: shew that

AB2 AC. AP+BD. BP.

16. Two circles intersect at B and C, and the two direct common tangents AE and DF are drawn: if the common chord is produced to meet the tangents at G and H, shew that GH2=AE2+ BC2.

17. If from a point P, without a circle, PM is drawn perpendicular to a diameter AB, and also a secant PCD, shew that

18. Three circles intersect at D, and their other points of intersection are A, B, C ; AD cuts the circle BDC at E, and EB, EC cut the circles ADB, ADC respectively at F and G: shew that the points F, A, G are collinear, and F, B, C, G concyclic.

19. A semicircle is described on a given diameter BC, and from B and C any two chords BE, CF are drawn intersecting within the semicircle at O; BF and CE are produced to meet at A: shew that the sum of the squares on AB, AC is equal to twice the square on the tangent from A together with the square on BC.

20. X and Y are two fixed points in the diameter of a circle equidistant from the centre C: through X any chord PXQ is drawn, and its extremities are joined to Y; shew that the sum of the squares on the sides of the triangle PYQ is constant. [See p. 147, Ex. 24.]

21.

PROBLEMS ON TANGENCY.

To describe a circle to pass through two given points and to touch a given straight line.

and from PD (or PC) cut off PQ equal to a side of this square.

Through A, B and Q describe a circle. Then since the rect. PA, PB = the sq. on PQ, .the ABQ touches CD at Q.

Ex. 4, p. 156.

III. 37.

Q. E. F.

NOTE. (i) Since PQ may be taken on either side of P, it is clear that there are in general two solutions of the problem.

(ii) When AB is parallel to the given line CD, the above method is not applicable. In this case a simple construction follows from III. 1, Cor. and III 16 and it will be found that only one solution exists.