Ex. 13. To trace the Logarithmic Spiral of Descartes from its equation

This curve is called also the Equiangular Spiral, because it cuts all its radii at the same angle: in fact

Ex. 14. To trace the Lituus from its equation

0

This curve touches Ox asymptotically, and approaches O by an infinite number of circumvolutions.

For methods of constructing geometrically the curves which have been above considered, and deducing their equations from their geometrical properties, which is the converse of the course which we have adopted, as well as for historical information respecting them, the student is referred to Peacock's Gregory's Examples.

Ex. 15. To trace the curve represented by the equation

x2 + y2 = a3xy.

or

It is evident that x and y must have the same sign: hence the curve can lie only in two quadrants. In the neighbourhood

of the origin, neglecting small quantities of higher orders than the second, we have

xy = 0,

which shews that the axes of x and y are both touched by a branch passing through the origin. If x and y be written for x and y, the equation is not altered, and therefore the curve is the same in both quadrants. It is impossible for either x or y to be infinite, since + y would then be a positive quantity of an infinitely higher order of magnitude than a2xy. The curve must therefore be of the form

Ex. 16. To trace the curve represented by the equation

At the origin there is

x* + y* = 2axy2.

xy2 = 0,

which shews that the axis of x is touched by two branches and the axis of y by one branch of the curve. It is evidently impossible for x to have a negative value. The curve is symmetrical with respect to the axis of x, because its equation remains the same when y is substituted for + y. Neither x nor y can be infinite, since x* + y1 would then be infinite compared with 2axy3.

Hence the curve must be of the form

C

Homogeneous Curves.

163. Curves represented by equations of the form

u = c,

where u is a homogeneous function of x and y, and where c is a constant quantity, may be traced very conveniently by assuming y = tx, and obtaining x and y in terms of t: a series of values must be assigned to t, and the corresponding values of x and y must be tabulated. It is desirable, however, first to ascertain whether there be any asymptotes by the method of Art. (110), Chap. II.

we obtain four asymptotes to the curve, represented by the

and therefore, observing that the ratio of y to x must be always of the same sign as t, we have the following table of corresponding values:

The form of the curve will therefore be the following.

Ex. 2. To trace the curve

x3y – 2x3y2 + xy3 = a*.

The Cycloid.

164. As an example of deducing the equation to a curve from its geometrical definition, which is exactly the converse of tracing a curve from its equation, we will investigate the equation to the cycloid from the nature of its generation.

Let C be the centre of a circle in contact at A with the straight line HK. Let O be the extremity of the diameter through A. Suppose this circle to roll, without sliding, along HK; the point 0 of the circumference will then trace out a curve OPK, which is called the Cycloid. Let OAx be taken as the axis of x, and Oy, at right angles to OA, as the axis of y. Suppose that, when O has arrived at a point P of the cycloid, the circle has revolved about its centre through an angle ; then its centre must have advanced, parallel to