EXAMPLES 1. Multiply the numerator and denominator of 5 by 7: this gives x =35 Ans. 35. 2. Multiply the numerator and denominator of 1: by 3, by 4, by 5, by 6, by 9, by 12, by 15, by 20. 3. Multiply each term of 125 by 7, by 8, by 12, by 14, by 15, by 17, by 45. § 85. It appears from Prop. II. that if the numerator of a fraction be divided by any number, the value of the fraction will be diminished as many times as there are units in the divisor. It also appears from Prop. IV. that if the denominator of a fraction be divided by any number, the value of the fraction will be increased as many times as there are units in the divisor. Therefore, when the numerator and denominator of a fraction are divided by the same number, the decrease from dividing the numerator will be just equal to the increase from dividing the denominator: hence we have, PROPOSITION VI. If the numerator and denominator of a fraction be divided by the same number, the value of the fraction will remain unchanged. Q. If the numerator of a fraction be divided by a number, how many times will the value of the fraction be diminished ? If the denominator be divided by the same number, how many times will the value of the fraction be increased ? If they are both divided by the same num. ber, will the value of the fraction be changed? Why not ? Repeat the proposition. EXAMPLES. 1. Divide both terms of the fraction by 4: this gives Ans. 4. 2. Divide each term by 8: this gives 88=1 3. Divide each term of the fraction 32 by 2, by 4, by 8, by 16, by 32. 4. Divide each term of the fraction on by 2, by 3, by 4, by 5, by 6, by 10, by 12, by 15, by 20, by 30, by 60. § 86. Any number greater than unity that will divide two or more numbers without a remainder is called their common divisor : and the greatest number that will so divide them, is called their GREATEST COMMON DIVISOR. EXAMPLES. 1. Take the two numbers 142 and 994. The greatest common divisor cannot be greater than the least number 142. This number will divide itself:-let us see if it will also divide 994. OPERATION. 142)142(1 The number 142 exactly divides itself, 142 giving a quotient of 1; it also divides 994 giving a quotient of 7. Therefore, 142 is the greatest common divisor. 142)9947 994 The number 2 and 71 are common divisors of the two numbers 142 and 994 since either of them will divide both of the numbers without a remainder. Two numbers may have several common divisors, but they have only one greatest common divisor. Q. What is the common divisor of two or more numbers ? What is their greatest common divisor? What is the difference between the common divisor and the greatest common divisor? What is the common divisor of 2 and 4 ? Of 4 and 6 ? What are the common divisors of 4 and 8? What is their greatest common divisor? What are the divisors of 12 and 16? Their greatest common divisor ? OPERATION. 2. Take the two numbers 72 and 90. Let us again see if the least number 72, is the 72)9001 72 greatest common divisor. After dividing we find a greatest common div. 18)72(4 remainder of 18. 72 Now if 18 will divide 72, it will also divide 90, for 90=72+18, and 18 will be contained once more in 90=72+18 than in 72: but 18 divides 72 without a remainder: therefore, 18 is the common divisor: hence we see that the common divisor of two numbers must also be a common divisor between the least number and the remainder after division. But 18 is the greatest common divisor; for, the greatest common divisor must be contained at least once more in 90 than in 72: hence, the greatest common divisor cannot be greater than the difference between the two numbers, which, in this case is 18. Therefore, we have PROPOSITION VII. The greatest common divisor of two numbers is obtained by dividing the greater by the less, then dividing the divisor by the remainder, and continuing to divide the last divisor by the last remainder until nothing remains. The last divisor will be the greatest common divisor sought. Q. Will the common divisor of two numbers divide their remainder after division? How do you find the greatest common divisor of two numbers? 3. Find the greatest common divisor of the two numbers 63 and 81. OPERATION. PROOF. 63)81(1 63 54 Greatest com. div. 9)18(2 9)63(7 63 81 18 4. Find the greatest common divisor of 315 and 405. Ans. 45. 5. What is the greatest common divisor of the two numbers 2205 and 2835 ? Ans. 6. Find the greatest common divisor of 1157 and 623. Ans. 7. Find the greatest common divisor of 792 and 1386. Ans. 198. Note:-If it be required to find the greatest common divisor of more than two numbers, find first the greatest common divisor of two of them, then of that common divisor and one of the remaining numbers, and so on, for all the numbers: the last common divisor will be the greatest common divisor of all the numbers. 8. What is the greatest common divisor of 246, 372, and 522 ? Ans. 9. What is the greatest common divisor of 492, 744 and 1044 ? Ans. 12. common LEAST COMMON MULTIPLE. § 87. A number is said to be a common multiple of two or more numbers, when it can be divided by each of them without a remainder. For example, 6 is a multiple of 2 and 3, because it is exactly divisible by each of them. So likewise, 12 is a common multiple of 2, 3, 4, and 6, because it is divisible by each of them. The least common multiple of two or more numbers, is the least number which they will separately divide without a remainder. For example, 12 is a common multiple of 2 and 3, but it is not the least common multiple, since 6 is also divisible by 2 and 3. Now 6 being the least number which is so divisible, it is the least common multiple of 2 and 3. To find the least common multiple of several numbers, we have the following RULE, I. Place the numbers on the same line, and divide by the least number that will divide two or more of them without a remainder, and set down in a line below the quotients and the undivided numbers. II. Divide as before, until there is no number greater than I that will exactly divide any two of the numbers: then multiply together the numbers of the lower line, and the divisors, and the product will be the least common multiple. If, in comparing the numbers together we find no common divisor, their product is the least common multiple. EXAMPLES. . . 1. Find the least common multiple of 3, 4 and 8. We first see, that 2 will OPERATION. divide 4 and 8, but as it 2)3 4 8 will not divide 3, we bring 2)3 2 down 3 into the 2nd line : 4 we again see that 2 is a 3 1 2 common divisor of 2 and Ans. 2x1x3X2X2=24. 4; and as there is no common divisor between any two of the numbers of the last line, it follows that 2x1x3 multiplied by the two divisors, is the least common multiple. . Q. When is one number said to be a common multiple of two or more numbers ? Of what numbers is 6 a common multiple? Of what numbers is 8 a common multiple? What is the least common multiple of two or more numbers ? "What is the difference between a common multiple, and the least common multiple? Give the rule for finding the least common multiple. If the numbers have no common divisor what is the least common multiple ? OPERATION. 9 3 . 2. Find the least common multiple of 3, 8, and 9. We arrange the numbers in a line and see that 3 will divide two 3)3. 8 of them. We then write down 1 8 the quotients 1, and 3, and also the 1x8X3X3=72. 8 which cannot be divided. Then as there is no common divisor between any two of the numbers, 1, 8, and 3, it follows that their product, multiplied by the divisor 3, will give the least common multiple sought. 3. Find the least common multiple of 6, 7, 8 and 10. Ans. 4. Find the least common multiple of 21 and 49. Ans. 147. 5. Find the least common multiple of 2, 7, 5, 6 and 8. Ans. 840. 6. Find the least common multiple of 4, 14, 28 and 98. Ans. 7. Find the least common multiple of 13 and 6. Ans. 78. 8. Find the least common multiple of 12, 4 and 7. Ans. 84. 9. Find the least common multiple of 6, 9, 4, 14 and 16. Ans. 1008. 10. Find the least common multiple of 13, 12 and 4. Ans. 11. What is the least common multiple of 11, 17, 19, 21 and 7 ? Ans. 74613 REDUCTION OF VULGAR FRACTIONS. § 88. Reduction of Vulgar Fractions is the method of changing their forms without altering their value. A fraction is said to be in its lowest terms, when there |