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the 6th order, but of the quadratic form, is deduced, which gives the values of mand n, and hence, m+nax='Vrt Vo? +9+. ra V+q> ;

3

3 or x =

of x= v=+,/777-7=tr vr

-r+

9

3

Cor. 1. In the given equation, if 39 is negative, and if 92 is lefs than 9 , this expression of the root involves impossible quantities ; while, at the same time, all the roots of that equation are possible. The reason is, that, in this method of solution, it is necessary to suppose that x the root may be divided into two parts, of which the product is

9.

But it is easy to fhew, that, in this, which is called the irreducible case, it cannot be done.

For example, the equation, (Ex. 3. Sect.

of this chapter), x-156x+560=o, belongs to the irreducible case, and the three roots are +4, +10, -14, and it is plain that none of tħete roots can be divided into two parts m and n, of which the product can be equal to -9=

= 52; for the 3

greatest

3.

156

greatest product from the division of thë greatest root--14, is -7X-7=49, less

than 52.

If the cube root of the compound surd can be extracted, the impossible parts balance each other, and the true root is, obtained.

The geometrical prob'em of the trisection of an arch is resolved algebraically, by a cubic equation of this form ; and hence the foundation of the rule for resolving an equation belonging to this case, by a table of fines.

Cor. 2. Biquadratic equations may be reduced to cubics, and may therefore be refolved by this rule.

Some other claffes of equations too, may be resolved by particular rules ; but these, and every other order of equations, are commonly resolved by the general rules, which may be equally applied to all.

II.

II. Solution of Equations, whose Roots are

commenfurate.

RULE I.

[ocr errors]

=o.

All the terms of the equation being brought

to one side, find all the divisors of the ab-
solute term, and

substitute them successively
in the equation for the unknown quantity.
That divifor which, substituted in this
manner, gives the result =o, shall be a
root of the equation. .
Ex. I. X-3ax2 + 2a’x-2a2b

- x2 + 3abx
The simple literal divisors of —2a2b, are
a, b, 2a, 2b, any of which may be inserted

Supposing x=ta, the equation becomes a-za+2a-2a-6 -ba? +3a-b

which is obviously=0. Ex. 2. *—2x2–33*+90=0.

x The divisors of go are 1, 2, 3, 5, 6, 9; 10, 15, 18, 30, 45, go.

The first of these divisors, which being inserted for x, will make the result =0, is

zd;

for x.

[ocr errors]

+3;

1

+3; +5 is another, and it is plain the last root must be negative, and it is -6.

When 3 is discovered to be a root, the given equation may be divided by x-3 =o, and the result will be a quadratic, which being resolved, will give the other two roots, +5 and 6.

The reason of the rule appears from the property of the absolute term formerly defined, viz. that it is the product of all the roots.

To avoid the inconvenience of trying many divisors, this method is shortened by the following

[blocks in formation]

Substitute in place of the unknown quantity

successively three or more terms of the progresion, 1, 0,-1, &c. and find all the divisors of the Jums that result, then take out all the arithmetical progresions that can be found among these divisors whose common aifference is I, and the values of x

will

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[ocr errors]

will be among those terms of the progres fons which are the divisors of the result arising from the substitution of x=o. Wben the series increases, the roots will be pofitive ; and when it decreases, the roots will be negative.

E X A M P L E.

Let it be required to find a root of the equan tion *--*?-10x+6=0.

o

[ocr errors]

-X

The operation is thus :

Supposit.

Result. Divisors. Ar. pro. x=+I

- 41,2,4, 4
o*3-*=10x+6=
.

+ 61,2,3,6, 3
(+141,2,7,14,2

XI

In this example there is only one progression, 4, 3, 2, and therefore 3 is a root, and it is -3, since the series decreases.

It is evident from the rules for transforming equations, (Chap. 2.), that by inserting for x, til=te) the result is the absolute term of an equation, of which the roots are

less

.

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