# The thing that started it 8.6 THE BINOMIAL THEOREM

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2 8.6 The Binomial Theorem 477 When we look at these expansions of a b n for n 1, 2, 3, 4, and 5, several patterns become apparent. 1. There are n 1 terms, from a n to b n. 2. Every term has essentially the same form: some coefficient times the product of a power of a times a power of b. 3. In each term the sum of the exponents on a and b is always n. 4. The powers (exponents) on a decrease, term by term, from n down to 0 where the last term is given by b n a 0 b n, and the exponents on b increase from 0 to n. Knowing the form of the terms in the expansion and that the sum of the powers is always n, we will have the entire expansion when we know how to calculate the coefficients of the terms. If we display the coefficients from the computations above, we find precisely the numbers in the first few rows of Pascal s triangle: Using the address notation for Pascal s triangle that we introduced in Section 8.4, the last row of coefficients in the triangle is 05, 15, 25, 35, 45, 55,andso we may write the expansion for a b 5 : a b 5 5 a 0 5 b a 4 b 5 1 2a 3 b 5 2 3a 2 b 3 5 4a 1 b 5 4 5a 0 b 5 Each term exhibits the same form. For n 5, each coefficient has the form r5, where r is also the exponent on b. For each term the sum of the exponents on a and b is always 5, so that when we have b r, we must also have a 5r.Finally,sincethe first term has r 0, the second term has r 1, etc., the r 1st term involves r. This leads to a general conjecture for the expansion of a b n which we state as a theorem that can be proved using mathematical induction. (See the end of this section.) Binomial theorem Suppose n is any positive integer. The expansion of a b n is given by a b n n 0a n b 0 n 1a n1 b 1 n ra nr b r n na 0 b n where the r 1st term is n ra nr b r,0rn. In summation notation, a b n r0 n n ra nr b r. (2) (1)

3 478 Chapter 8 Discrete Mathematics: Functions on the Set of Natural Numbers At this point, we have established only that the form of our conjecture is valid for the first five values of n, and we have not completely justified our use of the name Pascal s triangle of binomial coefficients. Nonetheless, the multiplication of a b 4 by a b to get the expansion for a b 5 contains all the essential ideas of the proof. We still lack a closed-form formula for the binomial coefficients. We know, for example, that the fourth term of the expansion of x 2y is 3 x 17 2y 3,butwe 20 cannot complete the calculation without the binomial coefficient 3.Thiswould require writing at least the first few terms of 20 rows of Pascal s triangle. Pascal himself posed and solved the problem of computing the entry at any given address within the triangle. He observed that to find rn,wecantakethe product of all the numbers from 1 through r, anddivideitintotheproductofthe same number of integers, from n downward. This leads to the following formula. Pascal s formula for binomial coefficients Suppose n is a positive integer and r is an integer that satisfies 0 r n. The binomial coefficient n r is given by n nn 1 nr1 r r We leave it to the reader to verify that the last factor in the numerator, n r 1,istherth number counting down from n. This gives the same number of factors in the numerator as in the denominator. (3) Strategy: We know Use Equation (3) to get the remaining coefficients. EXAMPLE 1 Using Pascal s formula Find the first five binomial coefficients on the tenth row of Pascal s triangle, and then give the first five terms of the expansion of a b 10. Follow the strategy , , , and Therefore the first five terms in the expansions of a b 10 are a 10 10a 9 b 45a 8 b 2 120a 7 b 3 210a 6 b 4. There is another very common formula for binomial coefficients that uses factorials. Equation (3) has a factorial in the denominator, and we can get a factorial in the numerator if we multiply numerator and denominator by the product of the rest of the integers from n r downto1: n r nn1...nr r nn1...nr1 r! nr nr n! r!nr!.

5 480 Chapter 8 Discrete Mathematics: Functions on the Set of Natural Numbers EXAMPLE 2 Symmetry in binomial coefficients Show that (a) (b) n r n n r. Follow the strategy. Strategy: Use Equation (4) to evaluate both sides of the given equations to show that the two sides of each equation are equal. (a) 6 2 (b) n r n n r 6! 2! 6 2! 6! 2! 4! n! r! n r! Thus n r n n r. and and 6 4 n! n r! n n r! n! n r! r! 6! 4! 6 4! 6! 4! 2!. EXAMPLE 3 Adding binomial coefficients Show that Get a common denominator and add fractions, but do not evaluate any of the factorials or binomial coefficients. Use Equation (3) to get 38 and 48, get common denominators, then add Thus Example 3 focuses more on the process than the particular result, hence the instruction to add fractions without evaluating. When we write out the binomial coefficients as fractions, we can identify the extra factors we need to get a common n denominator and then add. In Example 2, we proved that ( rn ) n r giving a symmetry property for the nth row of Pascal s triangle. Example 3 illustrates the essential steps to prove the following additivity property (see Exercise 61): n r n r 1 n 1 r 1

6 8.6 The Binomial Theorem 481 Symmetry and additivity properties Binomial coefficients have the following properties: Symmetry Additivity n r n (5) n r n r n r 1 n 1 (6) r 1 Notice that we used the additivity property from Equation (6) in Section 8.4 to get the n 1st row from the nth row in Pascal s triangle. This justifies our claim that the entries in Pascal s triangle are binomial coefficients. TECHNOLOGY TIP Evaluating binomial coefficients Most graphing calculators have the capacity to evaluate binomial coefficients directly, but we need to know where to look for the needed key. Most calculators use the notation ncr (meaning number of combinations taken r at a time, language from probability), and the key is located in a probability 20 (PRB, PROB) submenu under the MATH menu. To evaluate, say 4, the process is as follows. All TI-calculators: Having entered 20 on your screen, press MATH PRB ncr, which puts ncr on the screen. Then type 4 so you have 20nCr4. When you enter,the display should read All Casio calculators: Having entered 20 on your screen, press MATH PRB ncr, which puts on the screen. Then type 4 so you have 204. Whenyou execute, the display should read HP-38: Press MATH, go down to PROB, highlight COMB, OK. Then, on the command line you want COMB(20,4). Enter to evaluate. HP-48: Put 20 and 4 on the stack. As with many HP-48 operations, COMB (for combinations ) works with two numbers. MTH NXT PROB COMB returns EXAMPLE 4 Binomial Theorem Use the binomial theorem to write out the first five terms of the binomial expansion of x 2y 2 20 and simplify. Use Equation (1) with a x, b 2y 2, and n 20. The first five terms of x 2y 2 20 are x x 19 2y x 18 2y x 17 2y x 16 2y 2 4. Before simplifying, find the binomial coefficients, using either Equation (3) or the Technology Tip Therefore, the first five terms of x 2y 2 20 are x x 19 y x 18 y x 17 y x 16 y 8,or x 20 40x 19 y 2 760x 18 y x 17 y x 16 y 8.

7 482 Chapter 8 Discrete Mathematics Functions on the Set of Natural Numbers EXAMPLE 5 Finding a middle term In the expansion of 2x 2 1 x 10, find the middle term. There are 10 1or11termsintheexpansionofatenthpower,sothemiddleterm is the sixth (five before and five after). The sixth term is given by r x x x x x5 5 The middle term is 8064x 5. Strategy: First find the general term, then simplify. Finally, find the value of r 10 that gives 1 as the exponent of x. EXAMPLE 6 Finding a specified term In the expansion of (2x 2 x1 ) 10, find the term whose simplified form involves 1 x. Follow the strategy. The general term given in Equation (2) is r2x 2 10r 1 r x 10 r2 10r x 202r 1 r x r 10 r1 r 2 10r x 203r. For the term that involves 1 or x x1, find the value of r for which the exponent on x is 1: 20 3r 1, or r 7. The desired term is given by 10 72x x 1208x x 7 x. Proof of the Binomial Theorem We can use mathematical induction to prove that Equation (1) holds for every positive integer n. (a) For n 1, Equation (1) is a b 1 01 a 1 b 0 11 a 0 b 1 a b,soequation (1) is valid when n is 1. (b) Hypothesis: a b k k 0a k k 1a k1 b Conclusion: a b k1 k ra kr b k r kb k (7) k 1 a k 1 k1 a k b (8) 0 1 k 1 a k1r b r r k1b k1

8 8.6 The Binomial Theorem 483 Since a b k1 a b k a b a b k a a b k b, multiply the right side of Equation (7) by a,thenbyb, and add, combining like terms. It is also helpful k to replace 0k by 1 k1 0 and ( kk )by( k1 ), since all are equal to 1. a b k a b k 1 0 a k1 k 0 k 1a k b k 1 k a 2 k1 b 2 k r 1 k ra k1r b r k1b k1. Apply the additive property given in Equation (6) to the expressions in brackets to get Equation (8), as desired. Therefore, by the Principle of Mathematical Induction, Equation (1) is valid for every position integer n. EXERCISES 8.6 Check Your Understanding Exercises 1 6 True or False. Give reasons. 1. For every positive integer n, 3n! 3!n!. 2. There are ten terms in the expression of 1 x The middle term of the expansion of (x 1 x )8 is The expansion of x 2 2x 1 8 isthesameasthe expansion of x For every positive integer x,x 1 x4 x 2 1 x 4. Exercises 7 10 Fill in the blank so that the resulting statement is true. 7. After simplifying the expansion of x 2 1 x 5, the coefficient of x 4 is. 8. In the expansion of (x 1 x )6,themiddletermis The number of terms in the expansion of x 2 4x 4 12 is. Develop Mastery Exercises 1 14 Evaluate and simplify. Use Equations (3) (6). Then verify by calculator. 1. (a) 9 3 (b) (a) 14 3 (b) (a) 8 5 (b) (a) (b) (a) (b) (a) (b) (a) (b) (a) (b) (a) (b) (a) (b) (a) 10! 10! (b) 7! 7! 3! 12. (a) 8! 2! (b) 10! 13. (a) 6! 4! 3! (b) 8! 5! 3! 14. (a) 6! 3! (b) 6 3! Exercises Calculator Evaluation Use the Technology Tip to evaluate the expression. 15. (a) 8! (b) (a) 5! (b) (a) 8 3! (b) (a) (b) 8 3

9 484 Chapter 8 Discrete Mathematics Functions on the Set of Natural Numbers Exercises Evaluate and simplify. 19. n n n n n n 1! n 1! 23. n k 1 n k 24. n 1 n 1 r n r 1 Exercises Binomial Theorem Use the binomial theorem formula to expand the expression, then simplify your result. 25. x x 3y y x 29. 3x 1 25 x 28. x 2 2 x6 30. x 1 7 Exercises Expansion Use the formula in Equation (2). (a) Write the expansion in sigma form. (b) Expand and simplify x x y x 2 2 x5 34. x Exercises Number of Terms (a) How many terms are there in the expansion of the given expression? (b) If the answer in (a) is odd, then find the middle term. If it is even, find the two middle terms. 35. x x 2 1 x x x 2x 10 Find the first three terms in the expan- Exercises sion of 39. x 1 x x 3 x25 Exercises Find Specified Term If the expression is expanded using Equation (1), find the indicated term and simplify. 41. x 3 2 x5; thirdterm 42. x 2 2y 12; tenth term 43. 2x y 210; fourth term 44. x 1 2x 8 ; fourth term Exercises Specified Term If the expression is expanded and each term is simplified, find the coefficient of the term that contains the given power of x. See Example x 3 2 x x x x 2 2 x x 3 1 x x 2 3 x x 2 2x x 2 4x Exercises Solve Equation Find all positive integers n that satisfy the equation n! 2n! 54. 3n! 3!n! 55. 2n 2! n! 56. 3n! 3n 1! 57. n 3 n n 3 n n n (a) Show that ( 10 6 ) ( 10 7 ) ( 11 7 )bycarryingoutthe following steps. Using Equation (3), express each term of ( 10 6 ) ( 10 7 ) as a fraction with factorials; then, without expanding, get a common denominator and express the result as a fraction involving factorials. By Equation (3), show that the result is equal to ( 11 7 ). See Example 3. (b) Following a pattern similar to that described in part (a), prove the additivity property for the binomial coefficients n r n r 1 n 1 r By expanding the left- and right-hand sides, verify that n k 1 n k k 1 n k. 63. Explore Let S n 1 1!2 2! n n!1. S 1 1 1!122! and S 2 1 1!2 2!163! (a) Evaluate S 3, S 4,andS 5 andlookforapattern.on the basis of your data, guess the value of S 8. Verify your guess by evaluating S 8 directly. (b) Guess a formula for S n and use mathematical induction to prove that your formula is correct. 64. Explore Suppose f n x x 1 x 2n and let a n be the middle term of the expansion of f n x. (a) Find a 1, a 2, a 3,anda 4. (b) Guess a formula for the general term a n.isa n 2n! n!n!?

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