Binomial Coefficients Properties

11. Properties of Binomial Coefficients:

In (x + y )n or ( 1 + x )n, we know the binomial coefficients are :

nC0,nC1, nC2, nC3,….. nCrnCn.

In short, the binomial coefficients are also written as

C0, C1, C2, C3,….. Cr… Cn.

Let us now study a few salient properties of binomial coefficients:

Consider

( 1 + x )n = nc0 + nc1 . x + nc2 .x2 + nc3 . x3 +……..+ncn . xn………………(E)

To find the sum of the binomial coefficients nC0,nC1, nC2, nC3,….. nCrnCn in (E) above,

Put x = 1.

1. 2n = nc0 +nc1 + nc2 + …………. + ncn

2. Coefficients are odd or even based on the value of r.

If the r values of terms are odd, then binomial coefficients of such terms are called odd coefficients and if the r values of terms are even then binomial coefficients of such terms are called even coefficient. Therefore

C1, C3, C5 and so on are odd coefficients and C2, C4 , C6 and so on are even coefficients

In (1 + x )n =nc0 + nc0 . x + nc2 . x2 + ……… + ncn . xn

let us substitute -1 in x
( 1 – 1 )n = nc0 + nc1 . ( -1 ) + nc2 . (- 1 )2 + nc3 .(-1)3 + nc4 . (-1)4 + ……+ ncn .
(-1)n

0 = nc0 - nc1 + nc2nc3 + nc4 +…………..+ ncn (-1)n

Thus, nc0 + nc2 + nc4 +…………… = nc1 + nc3 + nc5 + ………….

2. c0 + c2 + c4 +……….. = c1 + c3 + c5 + ………….

Because the sum of the both the odd and even binomial coefficients is equal to 2n, so the sum of the odd coefficients = ½ (2n ) = 2n – 1 , and

Sum of the even binomial coefficients = ½ (2n) = 2n – 1

Thus, sum of the even coefficients is equal to the sum of odd coefficients.

In this way, we can derive several more properties of binomial coefficients by substituting suitable values for x and others in the binomial expansion.

12. Number of terms in the following expansions:

1. ( x + y )n is n + 1
2. ( x + y + z ) n is [( n + 1 ) ( n + 2 )]/2
3. ( x + y + z + w) n = [ ( n + 1)(n + 2 ) ( n + 3 )]/ 1. 2.3



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