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CHAPTER XI.

INDICES OR LOGARITHMS; AND MATHEMATICAL

TABLES.

Negative Symbols. If a>c, the symbol a c has an intelligible meaning. In this case we may easily show that

I. The addition of a-c (to any number whatever) is equivalent to the addition of a followed by the subtraction of c. Or, in symbols, +(a-c)= ta-c

.I. II. The subtraction of a - c (from any number>a) is equivalent to the subtraction of a followed by the addition of c. Or, in symbols, -(a-c)= -a+c

II. I. and II. here represent equivalences of operation.

But if a<c, the symbol a-c has no intelligible meaning by itself. It is convenient, however, to be able to use the above fundamental equivalences in all cases whatever.

Putting, then, a=0; (a -c) becomes (0 -c). Such a quantity is called a negative quantity. It may be written for brevity - c, or still better c. The sign is here called a sign of affection. It is useful to write it over a number to distinguish it from the sign – used to denote the operation of subtraction.

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Putting a=0 on the right-hand side of I. and II. (since the addition or subtraction of 0 has no effect), the right hand of (I.) becomes - c; that of II. becomes + C. Hence (I.) and (II.) become

+c= - cand -ċ= +c. In words :

The addition or subtraction of a negative symbol is interpreted to mean the subtraction or addition of the corresponding positive.

It remains to interpret multiplication involving a negative quantity.
If a>b and c>d, we may prove

that
(a - b)(c-d)=ac bc - ad+bd

.III. It is convenient to use this equation for all cases. Hence if a<b or c<d we so interpret multiplication of negatives that this shall always hold. Thus

Put b=0 and d=0; then ac=ac -0 -0+0=ac.
Put a=

O and d=0; then bc=0-bc-0+0=bc.
Put b=0 and c=0; then ad=0-0)- ad+0=ad.

Put a=0 and c=0; then bă=0-0-0+bd=bd. Thus the multiplication of a negative by a positive is negative ; and the multiplication of a negative by a negative is positive.

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229. If m is any positive integer, the symbol " is used to denote X X X X X X to m factors. Here ac is called the Base; m, the Index; and 2cm, the mth

Power

of x.

m

An mth Root of a given quantity means a quantity whose mth Power is equal to the given quantity.

The symbol m/x denotes an mth Root of x; i.e. ("/x)" = x.

230. To determine how many positive or negative roots a given positive or negative quantity has.

The rules for multiplying positive or negative quantities are I. The product of positive quantities is positive.

II. A change in sign of one factor changes the sign of the product.

m

-2

Hence a product containing an even number of negative factors is positive; and a product containing an odd number of negative factors is negative.

From this it follows conversely that (1) If m is odd, and a positive, there is no negative mth root of ac. (2) If m is odd, and x negative, there is no positive mth root of x.

(3) If m is even, and x negative, there is no positive and there is no negative mth root of x.

(4) There cannot be two different mth roots of a having the same sign. For, if possible, let y and z be two such roots, so that y" =zm = : then yM – 2* = (y-2) (3–1 + zym–+ zym-3 + ...... +2m-1) = 0. zm

xm But, by the rules of signs, every term in the second factor has the same sign, .-. this second factor cannot be zero. :: * -z = 0,

Hence the two roots supposed to be different are not different.

Thus, considering positive or negative values of m/x, we have the following results*:

If m is odd, and a positive, ma can have only one value, and this positive.

If m is odd, and x negative, mæ can have only one value, and this negative.

If m is even, and a positive, m/xc can have only two values, one positive and the other negative.

If m is even, and x negative, mc can have no values, positive or negative.

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231. The symbol m/s may be used for the present to denote the positive value of the mth root of x, if x is positive; and the negative value of the mth root of x, if x is negative (and m odd).

* Whether there are roots which are neither positive nor negative, is not here discussed.

232. The method of finding the arithmetical value of m/2, when m and a have any particular values, is not explained here.

But it is important to observe that its value cannot be exactly, but only approximately, determined in most cases.

A number which cannot be arithmetically expressed by a fraction having a finite integral numerator and denominator is called an Irrational or Incommensurable* number.

Thus, 32197=13 and is, therefore, rational. But y2=1.414 &c., cannot be exactly evaluated, and is therefore irrational.

Laws involving the same indices but different bases.

m

m

233. The power (or root) of a product (or quotient) of two quantities is equal to the product (or quotient] of the corresponding powers (or roots) of the two quantities. Thus

(1) (x * y) = xm x YM ;
(2) (3 + y) = 3c" ;
(3) m/(2 * y)=m/x x m/Y;

(4) m/(x + y) = "/w = /y.
For (oc x y)" means (2 x y) (* y) x ... to m factors

= (x x x x to m factors) ~ (y xyx to m factors)
= xm x ym.

....(1). Similarly (x + y)" = 2cm - ym....

...(2). In (1) write mac and my instead of x and y respectively.

y
Thus (m/ac *m/y)" = (m/x) * (/y)" = 2 x y.
.. taking the mth root of both sides
m/x m/y = m/(* y) ...

(3). Similarly m/= m/y="/(x + y)... ......... .(4)

=

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* That is, incommensurable with unity as explained in Art. 31.

Laws involving different indices but the same bases.

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Law I.

30m x 2" = xm+N. Law II. If m is greater than n, 2cm = = xm-n. Law III.

(acm)" =

= . Law IV. If m is divisible by n, */(2cm) = = 2min.

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235.

Law II. If m>n, 2cm - 2" -= 2cm-n. For, since m>n, m n is positive. .. in Law I., writing m-n instead of m, we have

cm-n x och = am-n+n= cm. Dividing both sides by ac", we have

acm-n = m = 2c".

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237. Law IV. If m is divisible by n, "(ac") = xm+.
For, since m is divisible by n, m= n is an integer.
.. in Law III., writing m+n instead of m, we have
(acm+n)" = xm+nxn

= 2". Taking the nth root of both sides

amun ^)(cc).

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