# Digital Lesson The Binomial Theorem

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Digital Lesson The Binomial Theorem

The binomial theorem provides a useful method for raising any binomial to a nonnegative integral power. Consider the patterns formed by expanding (x y)n. (x y)0 1 1 term (x y)1 x y 2 terms (x y)2 x2 2xy y2 3 terms (x y)3 x3 3x2y 3xy2 y3 4 terms 5 terms 6 terms (x y)5 x5 5x4y 10x3y2 10x2y3 5xy4 y5 (x y)4 x4 4x3y 6x2y2 4xy3 y4 Notice that each expansion has n 1 terms. Example: (x y)10 will have 10 1, or 11 terms. Copyright by Houghton Mifflin Company, Inc. All rights reserved. 2

Consider the patterns formed by expanding (x y)n. (x y)0 1 (x y)1 x y (x y)2 x2 2xy y2 (x y)3 x3 3x2y 3xy2 y3 (x y)4 x4 4x3y 6x2y2 4xy3 y4 (x y)5 x5 5x4y 10x3y2 10x2y3 5xy4 y5 1. The exponents on x decrease from n to 0. The exponents on y increase from 0 to n. 2. Each term is of degree n. Example: The 5th term of (x y)10 is a term with x6y4.” Copyright by Houghton Mifflin Company, Inc. All rights reserved. 3

The coefficients of the binomial expansion are called binomial coefficients. The coefficients have symmetry. (x y)5 1x5 5x4y 10x3y2 10x2y3 5xy4 1y5 The first and last coefficients are 1. The coefficients of the second and second to last terms are equal to n. Example: What are the last 2 terms of (x y)10 ? Since n 10, the last two terms are 10xy9 1y10. n The coefficient of xn–ryr in the expansion of (x y)n is written r or nCr . So, the last two terms of (x y)10 can be expressed 10 9 as 10C9 xy 10C10 y or as xy 9 9 10 Copyright by Houghton Mifflin Company, Inc. All rights reserved. 10 10 y . 10 4

The triangular arrangement of numbers below is called Pascal’s Triangle. 1 0th row 1 1 1st row 1 2 1 2nd row 1 3 3 1 3rd row 1 4 6 4 1 4th row 1 5 10 10 5 1 5th row 1 2 3 6 4 10 Each number in the interior of the triangle is the sum of the two numbers immediately above it. The numbers in the nth row of Pascal’s Triangle are the binomial coefficients for (x y)n . Copyright by Houghton Mifflin Company, Inc. All rights reserved. 5

Example: Use the fifth row of Pascal’s Triangle to generate the 6 , 6C4 and 6C2 . 5 6 sixth row and find the binomial coefficients 1 , 5th row 6th row 1 5 10 10 5 1 1 6 15 20 15 6 1 6 0 6 1 6 2 6 3 6 4 6 5 6 6 C0 6 6 C1 6 C2 6 6 1 6 C3 C4 6 C5 6 6 C6 6 and 6C4 15 6C2. 5 There is symmetry between binomial coefficients. C nCn–r n r Copyright by Houghton Mifflin Company, Inc. All rights reserved. 6

Example: Use Pascal’s Triangle to expand (2a b)4. 1 0th row 1 1 1st row 1 2 1 2nd row 1 3 3 1 3rd row 1 4 6 4 1 4th row (2a b)4 1(2a)4 4(2a)3b 6(2a)2b2 4(2a)b3 1b4 1(16a4) 4(8a3)b 6(4a2b2) 4(2a)b3 b4 16a4 32a3b 24a2b2 8ab3 b4 Copyright by Houghton Mifflin Company, Inc. All rights reserved. 7

The symbol n! (n factorial) denotes the product of the first n positive integers. 0! is defined to be 1. 1! 1 4! 4 3 2 1 24 6! 6 5 4 3 2 1 720 n! n(n – 1)(n – 2) 3 2 1 Formula for Binomial Coefficients For all nonnegative n! integers n and r, n Cr (n r )!r ! 7! 7! 7 Example: 7 C3 (7 3)! 3! 4! 3! 4! 3! (7 6 5 4) (3 2 1) 7 6 5 4 35 (4 3 2 1) (3 2 1) 4 3 2 1 Copyright by Houghton Mifflin Company, Inc. All rights reserved. 8

Example: Use the formula to calculate the binomial coefficients 50 12 C , C , and . 10 5 15 0 48 1 10! 10! (10 9 8 7 6) 5! 10 9 8 7 6 252 10 C5 (10 5)! 5! 5! 5! 5! 5! 5 4 3 2 1 10 C0 10! 10! 1! 1 1 (10 0)! 0! 10! 0! 0! 1 50! (50 49) 48! 50 49 50 50! 1225 2! 48! 2 1 48 (50 48)! 48! 2! 48! 12 12! 12 11! 12 12! 12 1 1 (12 1)! 1! 1! 1! 11! 1! Copyright by Houghton Mifflin Company, Inc. All rights reserved. 9

Binomial Theorem ( x y ) n x n nx n 1 y nCr x n r y r nxy n 1 y n n! with nCr (n r )!r ! Example: Use the Binomial Theorem to expand (x4 2)3. (x 4 2) 3 3 C0 (x 4 ) 3 3 C1( x 4 ) 2 (2) 3 C2(x 4 )(2) 2 3 C3(2) 3 1 (x 4 ) 3 3 ( x 4 ) 2 (2) 3 (x 4 )(2) 2 1 (2)3 x12 6 x 8 12 x 4 8 Copyright by Houghton Mifflin Company, Inc. All rights reserved. 10

Although the Binomial Theorem is stated for a binomial which is a sum of terms, it can also be used to expand a difference of terms. Simply rewrite (x y) n as (x (– y)) n and apply the theorem to this sum. Example: Use the Binomial Theorem to expand (3x – 4)4. (3 x 4) 4 (3 x ( 4)) 4 1(3 x) 4 4(3 x) 3 ( 4) 6(3 x) 2 ( 4) 2 4(3 x)( 4) 3 1( 4) 4 81x 4 4(27 x 3 )( 4) 6(9 x 2 )(16) 4(3 x)( 64) 256 81x 4 432 x 3 864 x 2 768 x 256 Copyright by Houghton Mifflin Company, Inc. All rights reserved. 11

Example: Use the Binomial Theorem to write the first three terms in the expansion of (2a b)12 . 12 12 12 12 11 (2a b) (2a) (2a ) b (2a)10 b 2 . 0 1 2 12 12! 12! 12! 12 12 11 11 (2 a ) (2 a )b (210 a10 )b 2 . (12 0)! 0! (12 1)! 1! (12 2)! 2! (212 a12 ) 12(211 a11 )b (12 11)(210 a10 )b 2 . 4096 a12 24576 a11b 135168 a10b 2 . Copyright by Houghton Mifflin Company, Inc. All rights reserved. 12

Example: Find the eighth term in the expansion of (x y)13 . Think of the first term of the expansion as x13y 0 . The power of y is 1 less than the number of the term in the expansion. The eighth term is 13C7 x 6 y7. 13! (13 12 11 10 9 8) 7! 13 C7 6! 7! 6! 7! 13 12 11 10 9 8 1716 6 5 4 3 21 Therefore, the eighth term of (x y)13 is 1716 x 6 y7. Copyright by Houghton Mifflin Company, Inc. All rights reserved. 13