The Derivative of a Product.

that when the variable is increased by the infinitesimal i, that the functions will become

u+U i and

+ Vi.

The product will therefore change from

to

น.

(u+Ui) (v + Vi) = u v + v Ui+u V i + U V 22, and the increase of the product is

v

U i + u Vi+U V i2;

the ratio of which to i is

v U + u V+ U Vi,

or, neglecting the infinitesimal U Vi, it is

v U + u V ;

that is, the derivative of a product of two functions is equal to the sum of the two products obtained by multiplying each function by the derivative of the other function.

177. Corollary. The derivative of

Solution of Numerical Equations.

SECTION III.

Numerical Equations.

178. Definition. A numerical equation is one all whose coefficients are given in numbers, so that it involves no literal expressions except those denoting the unknown quantities.

179. Problem. To solve a numerical equation. Solution. Let the equation be reduced as in arts. 105 and 118, to the form

u = 0.

A

Find by trial a value of the unknown quantity x which nearly satisfies this equation, and let this value be a; substitute this value in the given equation, and let the corresponding value of u be m. correction e in the value of a is then to be found, which shall reduce the value of u from m to zero. Now, if U is the derivative of u, and if M is the value of U which corresponds to

x = α,

M is, by art. 165, the rate at which u changes in comparison with x, so that when

By this means a value of x is found which is not

Rate of Approximation.

perfectly accurate, because M is not the rate at which u varies during the whole interval from

x = a to x = a + e;

but only while x differs infinitely little from a. Calling, therefore, a' this approximate value of x, we have

α'

m

M'

which may be used in the same way in which a was, in order to find a new approximate value a" of x ; and if m' and M' denote the corresponding values of u and U, we shall have

In the same way, may the approximation be continued to any degree of accuracy.

180. Problem. To determine the rate of approximation in the preceding solution.

Solution. This is a most important, practical point, and the determination of it will be found to facilitate the solution.

Now, it may be observed, that since e is the correction of a, its magnitude shows the degree of accuracy which belongs to a, and the accuracy of e is, obviously, the same with that of

a' = a + e.

The comparative accuracy of the approximate value of a, and the succeeding approximate value a', is, then, the same with the magnitude of e compared with the error of e.

Now, in determining e, M was supposed to be the rate at which u changed throughout the whole interval in the change of x from a to a + e. But if the rate of change of

Rate of Approximation.

M is denoted by N, that is, if N is the derivative of M, the value of M, at the end of this interval when x is a+e, must be increased to

M+ Ne.

In the middle of the interval when x is a +e, the value of M is

M+ 1 Ne,

which may be regarded as the average value of the rate of u's increase, throughout the interval. When x, therefore, becomes a + e, u becomes

which may, therefore, be regarded as the error of e; and its comparison with e gives the rate of approximation.

181. Corollary. If the value of a is accurate to a given place of decimals, as the gth, this will be shown by the magnitude of e, for we shall find

that is, a' is accurate to the (2 g + k)th place of decimals and the division of m by M may be carried to this extent.

182. Corollary.

the form

When the given equation has

u = h,

in which h is a given number, it may be brought to the form

while the value of the derivative is M, because h does not vary, and, therefore,