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Ex. 2. dy +2y x dx = 2a xü y3 dx.
The preceding is only a particular case of the following more general differential equation which is evidently capable of solution by the same process; viz. f'(4) Oh + f() $(x) = f(x).
SECTION 5.—The integration of partial differential equations
of the first order and degree. 384.] We must now consider differential expressions of another character; those, viz., wherein a relation is given between partial derived-functions and the variables : the general forms of these are (3) and (4) in Art. 364. I shall at present take the case wherein the partial derived-functions enter linearly, and where the coefficients are functions of the variables, including of course the case where they are constant.
First let it be observed, that a partial differential expression which arises from an implicit function of two variables of the form u = F(x, y) = 0,
(75) and the general form of which is
where P and Q are functions of x and y, although involving partial derived-functions, is in fact a total differential expression; for differentiating (75), we have
- dx = dy
(77) which is a total differential equation of the form (24).
Now, from the explanation of partial differential equations which has been given in Article 366, it follows that the integral of a partial differential equation of the first order and degree requires the introduction of an arbitrary function; and although the integral may be particular, yet it is not general without it. Since then a total differential equation of the form (77) may by an inversion of the process followed above be changed into a partial differential equation, so does its general integral require an arbitrary function: the method of determining it will be explained in Section 6 of the present Chapter: thereby also we shall be led to a solution of total differential equations still more general than that of the preceding Sections.
Let us now consider a partial differential expression of three variables x, y, z, and of the form
where P, Q, R are or may be functions of x, y and z, and in which z has been considered a variable dependent on two independent variables y and x. To discuss it however in the most general form, let us suppose the original function to be of the form u = F(x, y, z) = 0,
(79) where y denotes the arbitrary function, which the complete integral requires ; then, by the process of Art. 50, Vol. I,
substituting which in (78) we have
(81) and this is in the most general form of a partial differential equation of the first order and degree. It is the integral of this that we shall investigate.
PRICE, VOL. II.
Now of (79) the differential is
la ) dx + (4) dy + (44) dz = 0; from which and (81) we have
either of which equations, it will be observed, involves the other by reason of (83); and let us suppose two independent integrals of these equations to be found, and to contain two arbitrary constants C, and cz; and to be of the form fi(x, y, z) = (1, f2(x,y,z) = (y
(85) where c, and c, are arbitrary constants.
Then from these we have
on comparison of which with (81) it appears that either fı or fa satisfies (81): and so also will any arbitrary function of fi and fa: for let F represent an arbitrary function of fı and f, that is, of c, and cz; then multiplying the members of (87) severally by (de) and (dr ), and adding, we have
Plant) + 2 () +R () = 0; and therefore F($1,82) satisfies (81): and therefore we have for the general integral u = f(f1f2) = 0,2
= F(C1, C,) = 0;
or, as it may be expressed,
(89) fi = f(f); and either (88) or (89) is the general integral, because each contains an arbitrary function in its most general form.
385.] The process requires further development and illustration: but it will be better first to consider and solve some particular examples. Suppose the given equation to be (43) = f(x,y);
(90) then z = \f(x, y)dx++(y),
(91) where (y) is the arbitrary function which enters into the general integral, and which has y only for its subject. Similarly, if
(47) = f(x,y); 2 = \f(x, y)dy+(x).
Ex. 1. a(z) +0 () = c; or, what is equivalent, by means of the substitutions (80),
ales) +0 ( + c (en) = 0. (92) Now by the conditions (81) we have
.:. bx - ay = (1, cx— az = (z;
(94) and therefore by reason of (89), bx -- ay = f (cx — az);
(95) or u = f(bx — ay, cx — az) = 0);
(96) either of which is the general integral and involves an arbitrary functional symbol.
It is useful to observe the geometrical interpretation of the process :
Let (95) or (96) represent a surface: then from (92) it appears that the normal to the surface at every point of it is perpendicular to a straight line, whose direction-cosines are proportional to the quantities a, b, c: but as these determine the direction and not the position of a line, we can only conclude that every normal is perpendicular to one of a series of parallel straight lines : and the successive positions of these lines may vary according to any law; which law however is not given by the differential equation, but is required for the integral equation of the surface: in fact the insertion of it is absolutely necessary; for otherwise the equation cannot represent a surface; and the geometrical form of the law is the equation to the director curve along which the parallel straight line moves, and generates the surface; and this surface is cylindrical. This is also manifest from (88) and (94); (94) are the equations to two systems of parallel planes respectively parallel to the axes of z and y: and the intersection of two, viz., one of each system, will give the generating line of the surface; and the line of intersection will of course vary according to the functional relation between cı and cy, the particular values of which determine the particular intersecting planes.