CHAPTER VII.

THE PARABOLA AND HYPERBOLA.

51. As already stated, most of the properties of the hyperbola are the same as the corresponding properties of the ellipse, and proved by the same process, e being greater than 1. There are, however, some properties both of it and of the parabola which may be conveniently developed by a process more analogous to that of the Cartesian geometry. This process we shall develope presently. In the meantime we proceed to give a brief outline of the application to the parabola of the method employed in the preceding Chapter for the ellipse.

If p' be another point in the parabola, p'-p=ẞ, the limit to which approaches is a vector along the tangent; so that if xẞ=π-p, π is the vector to a point in the tangent; this gives

S (π − p) (øp + a ̄1) = 0 ...................... ..................(4);

so that op is a vector perpendicular to the axis.

reads by (6), 'vector along NP-SP-vector along AN', which requires that

NP = a❜pp

..(10),

.(11).

For the subtangent AT, put xa for π in (5), and there results

whence also the tangent bisects the angle SPQ; and SQ is perpendicular to and bisected by the tangent.

or (10) AY is parallel to, and equal to half of NP.

The locus of the middle points of parallel chords is thus found.

Let the chords be parallel to ẞ, π the vector of the middle point of one of the chords,

then

and

-1

S (π+xß) & (π+xß) + 2Sα ̄1 (π + xß) = 1;

which, since the term involving x must disappear, gives

a straight line perpendicular to pß, i.e. (6) parallel to the axis.

This equation may be written

Sẞ (þπ + a ̄1) = 0,

which shews (8) that the chords are perpendicular to the normal vector at the point where pπ, i. e. at the point where the locus of the chords meets the curve: in other words, the chords are parallel to the tangent at the extremity of the diameter which bisects them.

Ex. 1. If two chords be drawn always parallel to given lines, and cut one another at points either within or without the parabola,

the ratio of the rectangles of their segments is always the same whatever be their point of section.

Let POP, QOq be the chords drawn through O, and always parallel respectively to ẞ and y, which we will suppose to be unit vectors.

S (8+xß) (48 + $xß + 2a ̄1) = 1 ; .: x2Sß&ß+S848 + 2Sa ̄18 + Ax = 1,

the product of the two values of x being

COR. Let 0, 0' be the angles in which ẞ and y cut the axis; then since ẞ, y are unit vectors, if p be a vector to the parabola, drawn from S parallel to POp, which we may now call SP; p=nẞ, pp=(nẞ) = npß (44. 2),

Ex. 2. Find the locus of the point which divides a system of

parallel chords into segments whose product is constant.

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sine sin36'*