Thus one root lies between 4 and between 0 and

-

- 1, and the third lies between 2 and 3.

EXAMPLES XLVIII. g.

Determine the number and situation of the real roots of:

608. Two lines drawn at right angles to each other as in Fig. 1 form a simple system of lines of reference. Their

intersection, O, is called the origin. Distances from O along XX' are called abscissas; distances from XX'on a line parallel to YY' are called ordinates.

609. Abscissas measured to the right of the origin are considered positive, and to the left, negative. Ordinates measured above XX' are considered positive, and when taken below XX' are negative.

610. The abscissa and ordinate of a point are called the co-ordinates of that point, and the lines XX' and YY' are called Co-ordinate Axes, Axis of X, and Axis of Y, or Axis of Abscissas, and Axis of Ordinates.

611. Any point in the plane can be given by means of these co-ordinates: thus the point P of Fig. 1 is located by measuring the distance a to the right of O on the axis of abscissas, and then taking a distance b vertically upwards.

Since a and b can be either positive or negative, a point P' is found by taking a positive and b negative; P" is found by taking a negative and b negative; and P is found by taking a negative and b positive.

Abscissas and ordinates are generally represented by x and y respectively. Thus for the point P, x = a, and y = b; for P', x= a, and y = — b, etc.

612. Instead of writing "the point whose co-ordinates are 5 and 3," a more concise form is used: thus the point (5, 3) means that the point will be found by taking an abscissa of 5 units and an ordinate of 3.

Locate the points (3,2); (5, 8); (−4, 4); (— 8, − 3).

21

FIG. 2.

GRAPH OF A FUNCTION.

613. Let f(x) be any rational integral function of x, and let us place it equal to y. If we give a series of numerical values to x we can obtain corresponding -X values for y. Now laying off the values of x as abscissas, and the corresponding values of y as ordinates, we have a series of points which lie upon a line called the graph of the given function.

Ex. 1. Construct the graph of 2 x — 1.

Let 2x-1= y. Giving to x successive values, we obtain the corresponding values of y as follows:

The line ABCD, Fig. 4, is the required graph.

By taking other values between those assumed, we may locate

the curve with greater precision.

614. In giving values to x it is evident that the substitution of a root gives y=0, that is, the ordinate, or distance from the Axis of Abscissas, is 0; hence where the graph cuts the Axis of X we have the location of a real root, and the graph will cross the Axis of X as many times as the equation has real and unequal roots. If the roots of the equation be imaginary, the curve will not touch the Axis of X.

2. x2+2x+1. 4. x3+x−1. 6. x3_5x+3. 8. x8+3x2+5x-12.

SOLUTION OF HIGHER NUMERICAL EQUATIONS.

COMMENSURABLE ROOTS.

615. A real root which is either an integer or a fraction is said to be commensurable.

By Art. 582 we can transform an equation with fractional coefficients into another which has all of its coefficients integers, that of the first term being unity: hence we need consider only equations of this form. Such equations cannot have for a root a rational fraction in its lowest terms [Art. 575], therefore we have only to find the integral roots. By Art. 570 the last term of f(x) is divisible by every integral root, therefore to find the commensurable roots of f(x) it is only necessary to find the integral divisors of the last term and determine by trial which of them are roots.

616. Newton's Method. If the divisors are small numbers we may readily ascertain by actual substitution whether they are roots. In other cases we may use the method of Arts. 562 and 566 or the Method of Divisors, sometimes called Newton's Method.

Suppose a to be an integral root of the equation

By substitution we have

a"+P1an-1 +P2α-2+...+Pn-1α + Pn = 0.

Transposing and dividing throughout by a, we obtain

Pn

in which it is evident that must be an integer. Denot

Pn

a

ing by Q and transposing — Pn-19

а

Again, as before, the first member of the equation must be an integer. Denoting it by Q2 and proceeding as before, we must after n divisions obtain a result

Hence if a represents one of the integral divisors of the last term we have the following rule:

Divide the last term by a and add the coefficient of x to the quotient.

Divide this sum by a, and if the quotient is an integer add to it the coefficient of x2.

Proceed in this manner, and if a is a root of the equation each quotient will be an integer and the last quotient will be —1.

The advantage of Newton's method is that the obtaining of a fractional quotient at any point of the division shows at once that the divisor is not a root of the equation.

Ex. Find the integral roots of x4 + 4x3 x216 x 12 = 0. By Descartes' Rule the equation cannot have more than one positive root, nor more than three negative roots.

The integral divisors of 12 are 1, ± 2, ± 3, ± 4, ± 6. Substitution shows that 1 is a root, and that + 1 is not a root.