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Chapter, , 8, , BINOMIAL THEOREM, Mathematics is a most exact science and its conclusions are capable of, absolute proofs. – C.P. STEINMETZ , 8.1 Introduction, In earlier classes, we have learnt how to find the squares, and cubes of binomials like a + b and a – b. Using them, we, could evaluate the numerical values of numbers like, (98)2 = (100 – 2)2, (999)3 = (1000 – 1) 3, etc. However, for, higher powers like (98)5, (101)6, etc., the calculations become, difficult by using repeated multiplication. This difficulty was, overcome by a theorem known as binomial theorem. It gives, an easier way to expand (a + b)n, where n is an integer or a, rational number. In this Chapter, we study binomial theorem, for positive integral indices only., , 8.2 Binomial Theorem for Positive Integral Indices, , Blaise Pascal, (1623-1662), , Let us have a look at the following identities done earlier:, (a+ b)0 = 1, a+b≠0, (a+ b)1 = a + b, (a+ b) 2 = a 2 + 2ab + b 2, (a+ b) 3 = a 3 + 3a 2b + 3ab2 + b 3, (a+ b) 4 = (a + b) 3 (a + b) = a 4 + 4a3b + 6a2b 2 + 4ab3 + b 4, In these expansions, we observe that, (i) The total number of terms in the expansion is one more than the index. For, example, in the expansion of (a + b) 2 , number of terms is 3 whereas the index of, (a + b) 2 is 2., (ii) Powers of the first quantity ‘a’ go on decreasing by 1 whereas the powers of the, second quantity ‘b’ increase by 1, in the successive terms., (iii) In each term of the expansion, the sum of the indices of a and b is the same and, is equal to the index of a + b.

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BINOMIAL THEOREM, , 161, , We now arrange the coefficients in these expansions as follows (Fig 8.1):, , Fig 8.1, , Do we observe any pattern in this table that will help us to write the next row? Yes we, do. It can be seen that the addition of 1’s in the row for index 1 gives rise to 2 in the row, for index 2. The addition of 1, 2 and 2, 1 in the row for index 2, gives rise to 3 and 3 in, the row for index 3 and so on. Also, 1 is present at the beginning and at the end of each, row. This can be continued till any index of our interest., We can extend the pattern given in Fig 8.2 by writing a few more rows., , Fig 8.2, , Pascal’s Triangle, The structure given in Fig 8.2 looks like a triangle with 1 at the top vertex and running, down the two slanting sides. This array of numbers is known as Pascal’s triangle,, after the name of French mathematician Blaise Pascal. It is also known as Meru, Prastara by Pingla., Expansions for the higher powers of a binomial are also possible by using Pascal’s, triangle. Let us expand (2x + 3y)5 by using Pascal’s triangle. The row for index 5 is, 1, 5, 10, 10, 5, 1, Using this row and our observations (i), (ii) and (iii), we get, (2x + 3y) 5 = (2x) 5 + 5(2x) 4 (3y) + 10(2x) 3 (3y) 2 +10 (2x)2 (3y) 3 + 5(2x)(3y)4 +(3y) 5, = 32x5 + 240x4y + 720x3 y2 + 1080x2y3 + 810xy4 + 243y5.

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162, , MATHEMATICS, , Now, if we want to find the expansion of (2x + 3y) 12, we are first required to get, the row for index 12. This can be done by writing all the rows of the Pascal’s triangle, till index 12. This is a slightly lengthy process. The process, as you observe, will become, more difficult, if we need the expansions involving still larger powers., We thus try to find a rule that will help us to find the expansion of the binomial for, any power without writing all the rows of the Pascal’s triangle, that come before the, row of the desired index., For this, we make use of the concept of combinations studied earlier to rewrite, the numbers in the Pascal’s triangle. We know that, , n, , Cr , , n!, , 0 ≤ r ≤ n and, r!(n – r )!, , n is a non-negative integer. Also, n C0 = 1 = nCn, The Pascal’s triangle can now be rewritten as (Fig 8.3), , Fig 8.3, , Pascal’s triangle, , Observing this pattern, we can now write the row of the Pascal’s triangle for any index, without writing the earlier rows. For example, for the index 7 the row would be, C 0 7 C1 7C2 7C 3 7 C4 7C5 7C 6 7 C7., , 7, , Thus, using this row and the observations (i), (ii) and (iii), we have, (a + b) 7 = 7C 0 a7 + 7C1 a6b + 7C 2a5b 2 + 7C3a 4b 3 + 7C4a 3b4 + 7C5 a2b 5 + 7C 6ab6 + 7C7b 7, An expansion of a binomial to any positive integral index say n can now be visualised, using these observations. We are now in a position to write the expansion of a binomial, to any positive integral index.

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164, , MATHEMATICS, , Observations, n, , 1., , The notation, n, , n 0, , , , n, , k0, , C k a n k b k stands for, , n, , C 0a b + C1a n–1 b1 + ...+ nC ra n–rbr + ...+n Cna n–nbn, where b 0 = 1 = an–n., Hence the theorem can also be stated as, n, , (a  b ) n   n C k a n  k b k ., k 0, , 2., 3., 4., , 5., , n, , The coefficients Cr occuring in the binomial theorem are known as binomial, coefficients., There are (n+1) terms in the expansion of (a+b)n , i.e., one more than the index., In the successive terms of the expansion the index of a goes on decreasing by, unity. It is n in the first term, (n–1) in the second term, and so on ending with zero, in the last term. At the same time the index of b increases by unity, starting with, zero in the first term, 1 in the second and so on ending with n in the last term., In the expansion of (a+b) n, the sum of the indices of a and b is n + 0 = n in the, first term, (n – 1) + 1 = n in the second term and so on 0 + n = n in the last term., Thus, it can be seen that the sum of the indices of a and b is n in every term of the, expansion., , 8.2.2 Some special cases In the expansion of (a + b) n,, (i), , Taking a = x and b = – y, we obtain, (x – y)n = [x + (–y)]n, = nC0 xn + nC1xn – 1(–y) + n C2xn –2(–y) 2 + nC3 xn–3(–y)3 + ... + nC n (–y) n, = nC0xn – nC 1xn – 1y + nC 2xn – 2y2 – nC3xn – 3y3 + ... + (–1)n nCn yn, , Thus (x–y)n = nC 0xn – nC 1xn – 1 y + nC2xn – 2 y2 + ... + (–1)n nC n yn, Using this, we have, , (x–2y) 5 =, , 5, , C0x5 – 5C 1x4 (2y) + 5C2x3 (2y)2 – 5C3x2 (2y)3 +, , C 4 x(2y) 4 – 5C5 (2y)5, , 5, , = x5 –10x4 y + 40x3y2 – 80x2y3 + 80xy4 – 32y5., (ii), , Taking a = 1, b = x, we obtain, (1 + x) n = nC 0(1)n + nC1(1)n – 1x + nC2 (1)n – 2 x2 + ... + n Cnxn, = nC 0 + nC1x + nC 2x2 + nC3 x3 + ... + nC nxn, , Thus, , (1 + x) n = n C0 + nC1x + nC2x2 + nC 3x3 + ... + nC nxn

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BINOMIAL THEOREM, , 165, , In particular, for x = 1, we have, 2n = nC0 + nC1 + nC2 + ... + nC n., (iii), , Taking a = 1, b = – x, we obtain, (1– x)n, , = nC0 – n C1x + nC2x2 – ... + (– 1)n nC nxn, , In particular, for x = 1, we get, 0 = n C0 – nC1 + nC2 – ... + (–1)n nCn, 4, ,  2 3, Example 1 Expand  x   , x ≠ 0, x, , Solution By using binomial theorem, we have, 4, ,  3,  2 3,  3,  3,  3,  x   = 4C0 (x2)4 + 4C 1(x2)3   + 4C 2(x2 )2   + 4C 3(x2 )   + 4C4  , x, x, x, x, x, 2, , = x8 + 4.x6 ., , 3, , 4, , 3, 9, 27 81, + 6.x4 . 2 + 4.x2. 3 + 4, x, x, x, x, , = x8 + 12x5 + 54x2 +, , 108 81,  ., x x4, , Example 2 Compute (98)5., Solution We express 98 as the sum or difference of two numbers whose powers are, easier to calculate, and then use Binomial Theorem., Write 98 = 100 – 2, Therefore, (98)5 = (100 – 2)5, = 5C0 (100) 5 – 5 C1 (100) 4.2 + 5C 2 (100)3 22, – 5C 3 (100)2 (2)3 + 5 C4 (100) (2)4 – 5C5 (2) 5, = 10000000000 – 5 × 100000000 × 2 + 10 × 1000000 × 4 – 10 ×10000, × 8 + 5 × 100 × 16 – 32, = 10040008000 – 1000800032 = 9039207968., Example 3 Which is larger (1.01)1000000 or 10,000?, Solution Splitting 1.01 and using binomial theorem to write the first few terms we, have

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166, , MATHEMATICS, , (1.01)1000000 = (1 + 0.01)1000000, =, , 1000000, , C0 +, , 1000000, , C1(0.01) + other positive terms, , = 1 + 1000000 × 0.01 + other positive terms, = 1 + 10000 + other positive terms, > 10000, (1.01)1000000 > 10000, , Hence, , Example 4 Using binomial theorem, prove that 6n –5n always leaves remainder, 1 when divided by 25., Solution For two numbers a and b if we can find numbers q and r such that, a = bq + r, then we say that b divides a with q as quotient and r as remainder. Thus, in, order to show that 6n – 5n leaves remainder 1 when divided by 25, we prove that, 6n – 5n = 25k + 1, where k is some natural number., We have, (1 + a) n = nC0 + nC1 a + nC2a 2 + ... + nCna n, For a, , = 5, we get, (1 + 5) n = n C0 + nC1 5 + nC2 52 + ... + nCn 5n, , i.e., , (6)n = 1 + 5n + 52.nC 2 + 53.nC3 + ... + 5n, , i.e., , 6n – 5n = 1+52 (n C2 + n C35 + ... + 5n-2 ), , or, , 6n – 5n = 1+ 25 ( nC2 + 5 .nC3 + ... + 5n-2), , or, , 6n – 5n = 25k+1, , where k = nC2 + 5 .nC 3 + ... + 5n –2., , This shows that when divided by 25, 6n – 5n leaves remainder 1., , EXERCISE 8.1, Expand each of the expressions in Exercises 1 to 5., 5, , 1. (1–2x), , 5, , 2 x, 2.  – ,  x 2, , 3. (2x – 3)6

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BINOMIAL THEOREM, , 4., , x 1,   , 3 x, , 1, , 5.  x  , x , , , 5, , 167, , 6, , Using binomial theorem, evaluate each of the following:, 6. (96)3, 7. (102)5, 8. (101)4, 9. (99)5, 10. Using Binomial Theorem, indicate which number is larger (1.1)10000 or 1000., 11. Find (a + b) 4 – (a – b) 4. Hence, evaluate ( 3  2)4 – ( 3 –, , 2 )4 ., , 12. Find (x + 1)6 + (x – 1)6. Hence or otherwise evaluate ( 2 + 1)6 + ( 2 – 1)6., 13. Show that 9n +1 – 8n – 9 is divisible by 64, whenever n is a positive integer., n, , 14. Prove that, , 8.3, , 3, r 0, , r n, , C r  4n ., , General and Middle Terms, , 1., , In the binomial expansion for (a + b) n, we observe that the first term is, n, C 0an, the second term is n C1a n–1b, the third term is nC2 an–2 b2, and so on. Looking, at the pattern of the successive terms we can say that the (r + 1)th term is, n, C ra n–r br . The (r + 1)th term is also called the general term of the expansion, (a + b) n. It is denoted by Tr+1. Thus Tr+1 = nCr a n–rbr ., , 2., , Regarding the middle term in the expansion (a + b) n, we have, (i) If n is even, then the number of terms in the expansion will be n + 1. Since, ,  n  1 1,  , i.e.,, n is even so n + 1 is odd. Therefore, the middle term is , 2 , , th, , th, , n ,  1 term., 2 , th, , 8, , For example, in the expansion of (x + 2y) , the middle term is  1  i.e.,, 2, , 8, , 5th term., (ii) If n is odd, then n +1 is even, so there will be two middle terms in the

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168, , MATHEMATICS, , n 1 ,  n 1 ,  1  term. So in the expansion,  term and , expansion, namely, ,  2, ,  2 , th, , th, ,  7 1 ,  7 1 ,  1 , i.e., 5 th term., (2x – y) 7, the middle terms are ,  , i.e., 4 th and ,  2 ,  2, , th, , th, , 1, ,  2 n  1 1 ,  ,, In the expansion of  x   , where x ≠ 0, the middle term is , x, 2, , , , 2n, , 3., , th, , i.e., (n + 1)th term, as 2n is even., n, , 1, It is given by C nx   = 2nC n (constant)., x, 2n, , n, , This term is called the term independent of x or the constant term., Example 5 Find a if the 17th and 18 th terms of the expansion (2 + a) 50 are equal., Solution The (r + 1)th term of the expansion (x + y)n is given by Tr + 1 = nCr xn –ryr ., For the 17th term, we have, r + 1 = 17, i.e., r = 16, T17 = T16 + 1 = 50 C16 (2)50 – 16 a 16, , Therefore,, , = 50C16 234 a16., Similarly,, , T18 = 50C17 2 33 a 17, , Given that, , T17 = T18, , So, , C 16 (2)34 a 16 = 50C17 (2)33 a 17, , 50, , Therefore, 50, , i.e.,, , a=, , 50, , C 16 . 2, , 50, , C 17 .2 33, , C16  2, 50, , C 17, , =, , 34, , a 17,  16, a, , 50!, 17! . 33!, ,  2 =1, 16!34!, 50!, , Example 6 Show that the middle term in the expansion of (1+x ) 2 n is, , 1.3.5...(2n  1), 2n xn, where n is a positive integer., n!

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BINOMIAL THEOREM, , 169, , th, ,  2n , Solution As 2n is even, the middle term of the expansion (1 + x) is   1 ,,  2, , i.e., (n + 1)th term which is given by,, 2n, , Tn +1 = 2 nCn(1)2n – n(x)n = 2n Cnxn =, , (2n )! n, x, n! n !, , =, , 2n (2 n  1) (2n  2) ...4.3 .2 .1 n, x, n ! n!, , =, , 1.2.3.4...(2n  2) (2n  1) (2 n) n, x, n! n!, , =, , [1.3.5 ...( 2n – 1)][2.4.6...( 2n)] n, x, n! n!, , =, , [1.3.5...(2n  1)]2 n [1.2.3...n] n, x, n!n!, , =, , [1.3.5...(2n  1)] n ! n n, 2 .x, n! n!, , =, , 1.3.5...(2n  1) n n, 2 x, n!, , Example 7 Find the coefficient of x6y3 in the expansion of (x + 2y) 9., Solution Suppose x6y3 occurs in the (r + 1)th term of the expansion (x + 2y)9 ., Now, Tr+1 = 9Cr x9 – r (2y) r = 9Cr 2 r . x9 – r . y r ., Comparing the indices of x as well as y in x6y3 and in Tr + 1 , we get r = 3., Thus, the coefficient of x6y3 is, C3 2 3 =, , 9, , 9! 3 9 .8.7 3, .2 =, . 2 = 672., 3!6!, 3.2, , Example 8 The second, third and fourth terms in the binomial expansion (x + a) n are, 240, 720 and 1080, respectively. Find x, a and n., Solution Given that second term T2 = 240

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170, , MATHEMATICS, , We have, , T2 = nC 1xn – 1 . a, , So, , n, , Similarly, , n, , and, , n, , C 1xn–1 . a = 240, , ... (1), , C 2x, , 2, , ... (2), , C 3xn–3 a3 = 1080, , ... (3), , n–2, , a = 720, , Dividing (2) by (1), we get, n, , or, , (n  1)! a, . 6, (n  2)! x, , C2 xn  2 a 2 720, , i.e.,, n, C1 xn 1 a 240, , a, 6, , x ( n  1), , ... (4), , Dividing (3) by (2), we have, a, 9, , 2, x, (n  2), , ... (5), , From (4) and (5),, 6, 9, , ., n 1, 2 (n  2), Hence, from (1), 5x4a = 240, and from (4),, , Thus, n = 5, , a 3, , x 2, , Solving these equations for a and x, we get x = 2 and a = 3., Example 9 The coefficients of three consecutive terms in the expansion of (1 + a) n, are in the ratio1: 7 : 42. Find n., Solution Suppose the three consecutive terms in the expansion of (1 + a) n are, (r – 1)th, rth and (r + 1)th terms., The (r – 1)th term is nCr – 2 a r – 2, and its coefficient is nCr – 2. Similarly, the coefficients, of r th and (r + 1)th terms are nC r – 1 and nC r , respectively., Since the coefficients are in the ratio 1 : 7 : 42, so we have,, n, n, n, , and, , Cr 2, 1,  , i.e., n – 8r + 9 = 0, C r 1, 7, Cr 1, 7, , , i.e., n – 7r + 1 = 0, Cr, 42, , n, , Solving equations(1) and (2), we get, n = 55., , ... (1), , ... (2)

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BINOMIAL THEOREM, , 171, , EXERCISE 8.2, Find the coefficient of, 1. x5 in (x + 3) 8, , 2. a 5b7 in (a – 2b)12 ., , Write the general term in the expansion of, 4. (x2 – yx) 12, x ≠ 0., , 3. (x2 – y) 6, 5., , Find the 4th term in the expansion of (x – 2y) 12., , 6., , , 1 , Find the 13 term in the expansion of  9 x ,  , x  0., 3 x, , , 18, , th, , Find the middle terms in the expansions of, 7., , , x3 ,  3 , , 6 , , , 9., , In the expansion of (1 + a) m+n, prove that coefficients of a m and an are equal., , 7, , 10, , x, , 8.   9 y , 3, , , ., , 10. The coefficients of the (r – 1)th, rth and (r + 1)th terms in the expansion of (x + 1)n, are in the ratio 1 : 3 : 5. Find n and r., 11. Prove that the coefficient of xn in the expansion of (1 + x)2n is twice the coefficient, of xn in the expansion of (1 + x)2 n – 1., 12. Find a positive value of m for which the coefficient of x2 in the expansion, (1 + x) m is 6., , Miscellaneous Examples, 6, , 3 2, 1 , Example 10 Find the term independent of x in the expansion of  x ,  ., 3x , 2, 6r, , Solution We have Tr + 1, , 3 2 , = Cr  x , 2 , 6, ,  3, = Cr  ,  2, 6, , 6 r, , r, ,  1 ,  ,  3x , ,  , x2, , 6 r, ,   1, , r, , r, , 1  1 ,    r, x 3 

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172, , MATHEMATICS, , r, = (  1), , 6, , Cr, , (3)6  2 r 12  3 r, x, (2) 6  r, , The term will be independent of x if the index of x is zero, i.e., 12 – 3r = 0. Thus, r = 4, , (3) 6 8 5,  ., Hence 5th term is independent of x and is given by (– 1)4 6 C4, (2) 6 4 12, Example 11 If the coefficients of ar – 1, a r and ar + 1 in the expansion of (1 + a) n are in, arithmetic progression, prove that n2 – n(4r + 1) + 4r2 – 2 = 0., Solution The (r + 1)th term in the expansion is nCr ar . Thus it can be seen that ar occurs, in the (r + 1)th term, and its coefficient is n Cr . Hence the coefficients of ar – 1 , ar and, ar + 1 are n Cr – 1, nCr and n Cr + 1, respectively. Since these coefficients are in arithmetic, progression, so we have, nC r – 1+ nC r + 1 = 2. nCr . This gives, , n!, n!, n!, , 2 , (r  1)!( n  r  1)! ( r  1)!( n  r  1)!, r !(n  r )!, i.e., , 1, 1, , (r  1)!( n  r  1) ( n  r) (n  r  1)! (r  1) (r ) (r  1)!(n  r  1)!,  2, , or, , 1, ( r  1)! ( n  r  1)!,  2, , i.e., , 1, r (r  1)!(n  r ) (n  r  1)!, , 1, 1, , , , , (, n, –, r, ), (, n, r, 1), (, r, 1), (, r, ),  , , , , , 1, ( r  1)! ( n  r  1)![ r ( n – r )], , 1, 1, 2, , , ,, (n  r  1) ( n  r ) r (r  1) r ( n  r ), , or, , r( r  1)  (n  r ) ( n  r  1), 2, , (n  r ) ( n  r  1) r ( r  1) r (n  r ), , or, , r(r + 1) + (n – r) (n – r + 1) = 2 (r + 1) (n – r + 1), , or, , r2 + r + n 2 – nr + n – nr + r2 – r = 2(nr – r 2 + r + n – r + 1)

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174, , MATHEMATICS, , Thus, the coefficient of a 4 in the given product is – 438., Example 14 Find the r th term from the end in the expansion of (x + a) n., Solution There are (n + 1) terms in the expansion of (x + a)n . Observing the terms we, can say that the first term from the end is the last term, i.e., (n + 1)th term of the, expansion and n + 1 = (n + 1) – (1 – 1). The second term from the end is the nth term, of the expansion, and n = (n + 1) – (2 – 1). The third term from the end is the (n – 1)th, term of the expansion and n – 1 = (n + 1) – (3 – 1) and so on. Thus rth term from the, end will be term number (n + 1) – (r – 1) = (n – r + 2) of the expansion. And the, (n – r + 2)th term is nC n – r + 1 xr – 1 a n – r + 1., 18, , 3, 1 , Example 15 Find the term independent of x in the expansion of  x  3  , x > 0., 2 x, , Solution We have Tr + 1 =, , 18, , , , 18, , Cr, , Cr, ,  x, ,  1 ,  3 , 2 x, , 18 r, x 3, , 1, , 3, , 18 r, , ., r, , 2 .x, , r, 3, , =, , r, , 18, , 1, Cr r . x, 2, , 18 2 r, 3, , Since we have to find a term independent of x, i.e., term not having x, so take, We get r = 9. The required term is 18 C9, , 18  2 r, 0., 3, , 1, ., 29, , Example 16 The sum of the coefficients of the first three terms in the expansion of, m, , 3 , ,  x  2  , x ≠ 0, m being a natural number, is 559. Find the term of the expansion, x , , containing x3., 3 , , Solution The coefficients of the first three terms of  x  2 , x , , and 9 mC 2. Therefore, by the given condition, we have, m, , C0 –3 m C1+ 9 m C2 = 559, i.e., 1 – 3m +, , 9 m (m  1),  559, 2, , m, , are m C0, (–3) mC 1

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BINOMIAL THEOREM, , 175, , which gives m = 12 (m being a natural number)., r, , Now, ,  3 , 12, 12, –, r,   2  = 12C (– 3)r . x12 – 3r, Tr + 1 = C r x, r,  x , , Since we need the term containing x3, so put 12 – 3r = 3 i.e., r = 3., Thus, the required term is 12C3 (– 3) 3 x3, i.e., – 5940 x3., Example 17 If the coefficients of (r – 5)th and (2r – 1) th terms in the expansion of, (1 + x) 34 are equal, find r., Solution The coefficients of (r – 5)th and (2r – 1)th terms of the expansion (1 + x) 34, are 34C r – 6 and 34C 2r – 2, respectively. Since they are equal so 34 Cr – 6 = 34 C2r – 2, Therefore, either r – 6 = 2r – 2 or r–6 = 34 – (2r – 2), [Using the fact that if nCr = nCp, then either r = p or r = n – p], So, we get r = – 4 or r = 14. r being a natural number, r = – 4 is not possible., So, r = 14., , Miscellaneous Exercise on Chapter 8, 1. Find a, b and n in the expansion of (a + b) n if the first three terms of the expansion, are 729, 7290 and 30375, respectively., 2. Find a if the coefficients of x2 and x3 in the expansion of (3 + ax) 9 are equal., 3. Find the coefficient of x5 in the product (1 + 2x) 6 (1 – x) 7 using binomial theorem., 4. If a and b are distinct integers, prove that a – b is a factor of an – bn, whenever, n is a positive integer., [Hint write a n = (a – b + b) n and expand], 5. Evaluate, , , , 3 2, , , ,  , 6, , 3 2, , 2, 2, 6. Find the value of a  a  1, , , , 6, , ., ,   a, 4, , 2, , , , 4, ,  a2  1 ., , 7. Find an approximation of (0.99)5 using the first three terms of its expansion., 8. Find n, if the ratio of the fifth term from the beginning to the fifth term from the, n, , 4, 1 , end in the expansion of  2  4  is, 3, , , 6 :1 .

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176, , MATHEMATICS, , 4, , x 2, , 9. Expand using Binomial Theorem 1    , x  0 ., 2 x, , 2, 2, 10. Find the expansion of (3x – 2ax + 3a )3 using binomial theorem., , Summary, integral n is given by Binomial,  The expansion of a binomial forn anyn positive, n, n, n – 1, n, n – 2 2, , Theorem, which is (a + b) = C 0a + C 1a, b + C2a, b + ...+, nC, n, –, 1, n, n, a.b, + C nb ., n– 1, The, coefficients, of the expansions are arranged in an array. This array is, , called Pascal’s triangle.,  The general term of an expansion (a + b)n is Tr + 1 = nC ra n – r. br., th, , n , In, the, expansion, (a, +, b), ,, if, n, is, even,, then, the, middle, term, is, the,  1 , , 2 , n, ,  n 1 ,  n1 , term.If n is odd, then the middle terms are ,  and  1 terms.,  2 ,  2 , th, , th, , Historical Note, The ancient Indian mathematicians knew about the coefficients in the, expansions of (x + y)n , 0 ≤ n ≤ 7. The arrangement of these coefficients was in, the form of a diagram called Meru-Prastara, provided by Pingla in his book, Chhanda shastra (200B.C.). This triangular arrangement is also found in the, work of Chinese mathematician Chu-shi-kie in 1303. The term binomial coefficients, was first introduced by the German mathematician, Michael Stipel (1486-1567) in, approximately 1544. Bombelli (1572) also gave the coefficients in the expansion of, (a + b) n, for n = 1,2 ...,7 and Oughtred (1631) gave them for n = 1, 2,..., 10. The, arithmetic triangle, popularly known as Pascal’s triangle and similar to the MeruPrastara of Pingla was constructed by the French mathematician Blaise Pascal, (1623-1662) in 1665., The present form of the binomial theorem for integral values of n appeared in, Trate du triange arithmetic, written by Pascal and published posthumously in, 1665., , — —