2010/054) Show that in the expansion of the (a+b)^n

Show that in the expansion of the (a+b)^n, the sum of binomial coefficients of all odd terms is equal to the sum of he binomial coefficients of all even terms.

proof: this can be done algebraically
putting
a= 1, b= - 1

we see the sum = (1-1)^n = 0

all the odd terms(power of n) shall be -ve and all the even terms positive and hence both sums must be same for the total to be zero.

but this is not ineresting.

this can be done using method of combinotrics
now the coefficient of a^kb^n-k is the number of ways we can pick k objects out of n

now there are 2 cases

1) n is odd

we break n into two 2 parts k and n-k

if k is odd then n-k is even(number of ways k element can be selected is same as number of ways

so the number of ways odd numbers can be selected is same as the number of number of ways even number can be selected

so they are same.

2) n is even

for this case we pick one element and keep it aside. now n-1 is odd and the number of ways we pick odd elements is the number of ways we can pick even elements. now adding that to even element we do not change the number of ways but make odd elements and adding to odd elements we do not change the number of ways but make even elements

so by adding (n+1)st element number of odd selections is same as number of even selections and not adding the numbers are same.

hence number of odd element selection is same as number of even elements selection

hence proved