If the surds are not in their simplest form, it will save labor to reduce them to this form before multiplication. Example. The product of 5 V32, V48, 2V54 =5.472x4V3x2.3/6=480.v2. 73. Vo=480 x 6=2880. 264. To multiply surds of different orders: reduce them to equivalent surds of the same order, and proceed as before. Example. Multiply 5 32 by 2 v5. X 77 DIVISION OF SURDS. 265. Suppose it is required to find the numerical value of the quotient when 75 is divided by 17. At first sight it would seem that we must find the square root of 5, which is 2.236..., and then the square root of 7, which is 2:645..., and finally divide 2.236... by 2:645...; three troublesome operations. But we may avoid much of this labour by multiplying both numerator and denominator by 27, so as to make the denominator a rational quantity. Thus 15 15 17 15x7 133 7 7 Now 135=5.916... = .845... 17 7 266. The great utility of this artifice in calculating the numerical value of surd fractions suggests its convenience in the case of all surd fractions, even where numerical values are not required. Thus it is usual to simplify as follows: Vc adb adbono adobe dc Vexno The process by which surds are removed from the denominator of any fraction is known as rationalizing the denominator. It is effected by multiplying both numerator and denominator by any factor which renders the denominator rational. We shall return to this point in Art. 270. anb с 3 V 2a* 267. The division of surds then may be effected by expressing the result as a fraction, and rationalizing the denominator. Example 1. Divide 4,/75 by 25/56. 41/75 The quotient= 4x5/3 2./3 25 256 25 x 2,145/14 2/3 x 714 2/42 142 5714x7145 x 14 35 36 Yox Yo _ Voe_ Voc Example 2. Yo Yoʻx/C uc EXAMPLES XXXII. c. Find the value of 1. 214 x 21. 2. 3/8 x 76. 3. 5 Jax 2 /3. 4. 2/15 3/5. 5. 8/12 x 3/24, 6. 4.2+2x9c-2. 7. 21/384-8/98. 8. 5./27-3/24. 9. – 13/125:-5./65. 10. 3168 x 147. 11. 53128 x 2 3432. 12. 6714;2./21. 3/11 5 3/48 6/84 13. a/63 x 62 Ja. 14. 15. 2/98 7/22 57112" 1392 3 18.23 16. . 17. с a 14 10 22. Given V2=1:41421, 13=1.73205, V5=2.23607, V6=2.44949, 17=2.64575: find to four places of decimals the numerical value of 25 48 21. 76 1 2/3 256 26. 27. 28. 29. 1575 268. Hitherto we have confined our attention to simple surds, such as 15, Ya, Vx+y. An expression involving two or more simple surds is called a compound surd; thus 27a-3./6; Ya+b are compound surds. 269. The multiplication of compound surds is performed like the multiplication of compound algebraical expressions. Example 1. Multiply 2 x – 5 by 3/x. = 6x – 15/x. Example 2. Multiply 2.15+31x by 15 - Nx. =25.5+3/5./x-2/5.7x-3.13.0/x =10 – 3x+15. = 4x + 7 – 4.0 + 4/72 - 4.x? EXAMPLES XXXII. d. Find the value of 1. (3 78-5) 2/x. 2. (1x-Ja) x 2Jx. 3. (Ja+)x Wab. 4 (4x+y-1)x /+y. 5. (2/3+372) 6. (17+5/3)(2/7-4/3). 7. (3.75 -4./2)(2.75 +372). 8. (3/a-2.1.x) (2 Ja+3/x). 9. (Vx+Vx - 1)x13 - 1 . 10. (Na+a-vx- a) x Virta. 11. (Na+x– 2. Ja)? 12. (2.Ja-V1+4a).. 13. (a+x-Wa-2) 14. (Va+x-2)(a+x-1). 15. (J2+73-75)(2+3+w/5). 16. (15+3/2+7) (15+3/2-77). Write down the square of 17. 2u+a-2.0-a. 18. Væ? – 2y2 +1.22 +2y2. 19. vm+1+1m-n. 20. 31a +69_2142–62. 21. 3x/2-37- 2x? 22. 142° +1 -14.x2 - 1 270. One case of the multiplication of compound surds deserves careful attention. For if we multiply together the sum and the difference of any two quadratic surds we obtain a rational product. Examples. (1) Wa+b) (a/b)=(a)-(b)=a-b. (2) (3/5+42/3) (3/5 – 4,/3)=(3,5)2 – (4/3)2 = 45 – 48=-3. Similarly, (4- Va+b) (4 + wa+b)=(4)? - (a+b)2=16 - a - b. 271. DEFINITION. When two binomial quadratic surds differ only in the sign which connects their terms they are said to be conjugate. Thus 3/7+5/11 is conjugate to 3/7-5/11. =(3/a)2-(Vx-9a)?=9a-(c-9a)=18a-2. Division OF COMPOUND SURDS. 272. If the divisor is a binomial quadratic surd, express the division by means of a fraction, and rationalize the denominator by multiplying numerator and denominator by the surd which is conjugate to the divisor. Example 1. Divide 4+3,/2 by 5 - 312. 4+3./2_4+372 5+3/2 7 х Example 2. Rationalize the denominator of Va+62+ a 62 Vas +2°-a The expression Na+ +62 +a Va? +62 - a (a2 +69) - a? by Х 13+12 7+42/3 Example 3. Divide 2-3 :- 2 13+2 N3 – 12 The quotient 2-03 7+423 (73)2-(12) 1 87 Example 4. Given N5=2•236068, find the value of 7-2750 Rationalizing the denominator, 87 87 (7 +25) 7 -- 2/5 49 - 20 =3 (7+2,75) = 34.416408. It will be seen that by rationalizing the denominator we have avoided the use of a divisor consisting of 7 figures. 273. In a similar manner, where the denominator involves three quadratic surds, we may by two operations render that denominator rational. Example. V2 2(V2+ 3+15) The expression (V2+V3 -- V5)(V2+ V3+V5) 276 (26) 76 6 |
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