6(c— af) NOTE. — The quantity within the parentheses must be taken as a whole (2 27). In Exercise 23 the sum of a and b, indicated by (a + b), is the unit; in Exercise 24 the difference of a? and c, indicated by (a? - c?), is the unit. 26. Add 3a(gʻ – h?) – 2a(gʻ -- ha) + 4a(go — ha) +8a(g? — h?) – 2a(go — h). Ans. 11a(g– h). 27. Add 3c(a'c — 6?) — 9c(a’c — 62) — 7c(a’c — 62) + 15c(a’c – 6?) + c(a’c — b?). Ans. 3c(a’c — 6?). 34. In algebra the term “add " does not always, as in arithmetic, convey the idea of augmentation; nor the term “sum ” the idea of a number numerically greater than any of the numbers added : for, if to a we add – b, we have a – b, which is, arithmetically speaking, a difference between the number of units expressed by a and the number of units expressed by b; consequently this result is numerically less than a. . To distinguish this sum from an arithmetical sum, it is called the algebraic sum. SUBTRACTION. 35. Subtraction is the operation of finding the difference between two algebraic quantities. 36. The subtrahend is the quantity to be subtracted; the minuend, the quantity from which it is taken. 37. The difference of two quantities is such a quantity as, added to the subtrahend, will give a sum equal to the minuend. (1) From 17 a take 6 a. In this example, 17a is the minuend, and 6 a the subtra- 17 a hend. The difference is 11 a, because 11 a added to 6 a gives 6a 17a. lla Note. — The difference may be expressed by writing the quantities thus : 17a – 6a = 11a, in which the sign of the subtrahend is. changed from + to (2) From 15x take - 92. The difference, or remainder, is such a quantity as, being 150 added to the subtrahend (- 9x), will give the minuend (15x). - 9.x That quantity is 24x, and may be found by simply changing the sign of the subtrahend, and adding: whence we may 24 x write, 15% - (- 9x) = 24 x. (3) From 10 ax take a - b. The difference, or remainder, is such a quantity as, added to a – b, will give the minuend (10 ax). What is that quantity ? If you change the signs of both terms of the subtra- 10 ax hend, and add, you have 10 ax – a + b. Is this the t atrue remainder ? Certainly : for, if you add the remainder to the subtrahend (a - b), you obtain the 10 ax – a +bi minuend (10 ax). ta-6 *It is plain, that if you change the signs of all the 10 ar terms of the subtrahend, and then add them to the minuend, and to this result add the given subtrahend, the last sum can be no other than the given minuend: hence the first result is the true difference, or remainder (8 37). From the preceding examples we have, for the subtraction of algebraic quantities, the following rule: Write the terms of the subtrahend under those of the minuend, placing similar terms in the same column. Conceive the signs of all the terms of the subtrahend to be changed from + to – or from – to t, and then proceed as in addition. Exercises. Subtract the following: 1. 3 ab 4. 16 a?l?c . 7. . 3 ax . 9abc 3ax – 80 3 a363c . 3 ας 14 a?b%c .::.... 3. 9abc .....6. 24'a’b?x . 9. 2am 7 abc . 7 a2b2x ах 2 abc . .. 17 a’b’x ... 2 am .- ax 8c 2. 6 ax 3 ax 9 ac 4 abx -- 9 ac |