Imágenes de páginas
PDF
EPUB

by the rules of multiplication, is +aa or +a; but if we involve -a to the 3d power, we have (—a)×(—a)×(—a), which, by the same rules, is -aaa or—a3. It is evident that the 4th power of a is +a+; the 5th power of a is —a3, &c. ; the even powers being positive and the uneven powers negative. We shall thus find that (—a)2o=+a2o, and (-a)31--aa1, &c. &c.

20

15. We see then that the squares of +a and —a are both +a3. If then it be required to extract the square root of +a2, this root may be either +a or -a; so that the result is ambiguous, which is expressed by prefixing both signs thus, a; that is,

a±a.

But in extracting the cube root no ambiguity will exist, for the cube of +a and the cube of -a are not the same, the one being +a3 and the other —a3; and, consequently, the cube root of +a3 is +a, and the cube root of —a3 is · -α. In the same way the 4th root of a will be either a ora, but the 5th root of +a5 will be +a, and the 5th root of —ɑ5 will be —a.

Hence the rule, the even root of a positive quantity is either positive or negative; the uneven root of a positive quantity is positive; and the uneven root of a negative quantity is negative.

16. The even root of a negative quantity is impossible. For the square root of —a2 is neither a nor―a; since +a squared is +a, and a squared is +a; and there is no quantity which, multiplied by itself, will produce —a3. For the same reason 4/—a,

8

—,—1 are impossible. These quantities, or rather expressions, are called imaginary. It is usual to reduce them as fol

lows:

E

To find the square root of -a3, we have √—a3=√a3×(−1)= ±a-1, (Arts. 11 and 15,) in which last form the imaginary part is the even root of -1; and to this form all imaginary expressions may be reduced. Thus,

[blocks in formation]
[blocks in formation]

is a possible quantity, being the even root of a

[blocks in formation]

1. Find the product of -"y"z", x"Y3z",—X"Y"2".

Ans. xm+n+y+ntezm+nte or (xyz)m+nte.
Ans. (xm-ny-I—m)P.

2. Divide xyz" by x11уz.

3. Divide -axbxx by —ɑnуbuz.

4. Involve -b to the nth power.

5. Involve -b to the (2n)th power.

Ans. (ab).

Ans. +b ifn is even, —b" if n is uneven.

Ans. +62 (for 2n is even whatever be the value of n.)

6. Involve -b to the (2n+1)th power.

Ans. -b3n+1 (for whatever be the value of n, 2n is even, and 2n+1 is uneven.)

[merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small]

CHAPTER II.

EXPONENTS IN GENERAL.

18. In the preceding chapter we have applied the rules for the multiplication, division, &c. of powers only to those cases in which the exponents were whole positive numbers. Exponents, however, may be also fractional and negative.

3

3

19. Let it be required to extract the 3d root of a2. We have seen that this would be a3, but algebraists have agreed to apply to such cases the general rule for the evolution of powers given in Art. 9. To extract the 3d root of a2, according to this rule, we divide the exponent by 3, which gives a3; therefore, a3 and a are equivalent expressions, both signifying "the 3d root of a squared ;" and in the fractional form, the numerator of the fraction indicates the power, and the denominator the root. In the same way we find ✅a=a*; ↓ã=a3; √a°—a3; Van=am, &c.

3

5

n

The use of fractional exponents thus renders the radical sign unnecessary, and enables us to express roots and powers by the same general mode of notation.

20. Again, let it be required to divide a5 by a7. This is,

a5

which

[ocr errors]

fraction reduced by dividing its numerator and denominator by a5,

1

a3

becomes But by Art. 6, powers of the same quantity are divided by subtracting their exponents. Hence, to divide a5 by a7, we must subtract 7 from 5; but 5-7-2, so that the quotient 1 will be a with the exponent -2, or a-2. Therefore, and a-2 are equivalent expressions; but the latter has the advantage of representing a fraction in the same form with whole numbers. In the same way

[blocks in formation]
[ocr errors]

1

We see then that the negative exponent indicates the reciprocal of a power; that is, a- is the reciprocal of a; a is the reciprocal of a1; a―m is the reciprocal of am, &c.

21. We apply the rules given in the preceding chapter also to fractional and negative exponents. Thus, to multiply a by αξ we add the exponents (Art. 5,) and we have

[ocr errors][merged small][merged small][merged small][merged small][ocr errors][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small]

22. Applying the rule for division of powers, (Art. 6,) we have

[ocr errors][ocr errors][merged small][merged small][merged small][merged small][subsumed][subsumed][ocr errors][subsumed][merged small][ocr errors][merged small][subsumed][ocr errors]

23. Applying the rule for involution of powers, (Art. 8,) we have

[blocks in formation]

24. Applying the rule for evolution of powers, (Art. 9,) we have

[subsumed][subsumed][ocr errors][subsumed][merged small][merged small][merged small][merged small]
[ocr errors]

25. By using fractional exponents involution and evolution are performed by the same rule; that is, by multiplying the exponents. Thus, the above examples will be performed,

3

8a=(8a3) (Art. 19,) =83aš×3 (Art. 8,)=2a+

[ocr errors][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][ocr errors][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][subsumed]

26. We conclude this chapter by exhibiting the regular series of powers of a, decreasing by unity. Beginning at am and dividing continually by a we have the following series:

am,

am m-1 am. a3, a3, a1‚ ao, a ̄1‚ a ̄ ̄3, a ̄3,... a ̄m+3, a¬m+1, a—m.

[ocr errors]

which is a regular series of powers of a from +m tom. Any two terms equally distant from ao are the reciprocals of each other; thus, a1 is the reciprocal of a1, a3 of a-2, &c., and am of a-m.

The term ao is found (as the other terms are found) by subtract

« AnteriorContinuar »