intersect CA in E; intersect AB in F;

CF, the fourth harmonic to CA, CP, CB, then D, E, F lie in the same straight line.

Let f, g, h be the co-ordinates of P. Then the equation

of AP is

From the form of these equations it will be seen that the straight line

passes through D, E, F. Hence these three points are in a straight line.

COR. The converse proposition to that above enunciated may be demonstrated by similar reasoning.

The point P, and the line DEF, may be called harmonics of one another with respect to the triangle ABC.

By combining the proposition last proved with that proved in Art. (22), we shall obtain a demonstration of the statements made in Art. 6; that the points in which the external bisectors of each angle of a triangle respectively intersect the sides opposite to them, lie in the same straight line; and that the points in which the external bisector of any one angle and the internal bisectors of the other two angles, intersect the sides respectively opposite to them, lie in the same straight line.

These straight lines will be respectively represented by the equations,

26. DEFS.

and let

a+B+y=0, B+y-α=0,

y+a-B=0, a+B-y=0.

ON INVOLUTION.

Let O be a point in a given straight line,
P, P', Q, Q', R, R'......

be a series of points on that line so taken that

OP. OP' = OQ. OQ' OR. OR' = ......

=

= a constant k2 suppose.

Then these points are said to form a system in involution.

If K be a point such that OK2 = k2, K is called a focus of the system.

If 2 be positive, there will evidently be two such foci, one on each side of O, if negative (and k therefore imaginary) there will be no real foci.

The point O is called the centre of the system.

Two points, such as P, P', are said to be conjugate to one another.

It is evident that each focus is conjugate to itself, and that the conjugate of the centre is at an infinite distance, and that a point and its conjugate will be on the same, or different sides of the centre, according as the foci are real or imaginary.

The system will be determined when two foci, or a centre and focus, are given. It will also be determined if two pair of conjugate points be given; as may be seen as follows.

Let p, p, q, q' be the respective distances of the four points from any arbitrary point on the line, x the distance of the centre from the same point.

27. PROP. The anharmonic ratio of four points is equal

to that of their four conjugates.

For, if OP=p, OQ=q,

then [PQRS] =

OR=r, OS=s,

(q − p) (s − r)

=

(s-p) (r−q)'

(k2 k2 (k2 k2

(p − s) (q − r)

= [PQRS],

COR. It is evident that [PQRP'] = [P'Q'R'P].

28. PROP. Any two conjugate points form, with the two foci, an harmonic range.

.. K ̧P.K‚P' = K1P'.K„P,

or the four points in question form a harmonic range.

Conversely, if there be a system of pairs of points in a straight line, such that each pair forms, with two given points, an harmonic range, the aggregate of the pairs of points will form a system in involution, of which the two given points are the foci.

29. A system of straight lines, intersecting in a point, may be treated in the same manner as a system of points lying in a straight line, the sine of the angle between any two lines taking the place of the mutual distance of two points. It is not difficult to see that, if a system of straight lines in involution be cut by a transversal, the points of section will also be in involution.

33

CHAPTER II.

SPECIAL FORMS OF THE EQUATION OF THE SECOND DEGREE.

1. We now proceed to the discussion of the curve represented by the equation of the second degree. We shall first prove that every curve, represented by such an equation, is what is commonly called a conic section; and then, before proceeding further with the consideration of the general equation, shall investigate the nature of the curve corresponding to certain special forms of the equation.

PROP. Every curve represented by an equation of the second degree is cut by a straight line in two points, real, coincident, or imaginary.

The general equation of the second degree is represented by

ua2 + vß2 + wy2 + 2u' By + 2v' ya +2w' aß = 0.

To find where the curve, of which this is the equation, is cut by the straight line

la + mB+ny = 0,

we may eliminate a between the two equations. This will give us a quadratic for the determination of, to each of the

two values of this ratio, real, equal, or imaginary, one value of a will correspond; whence it appears that the straight line and the curve cut one another in two real, coincident, or maginary points.

Hence, the curve is of the same nature as that represented by the equation of the second degree in Cartesian co-ordinates, and is, therefore, a conic section.