NCERT Solutions
Class 11 Maths
Binomial Theorem

Ex.8.2 Q.9
In the expansion of (1 + a) m + n, prove that coefficients of am and an are equal.
It is known that (r + 1) th term, (Tr+1), in the binomial expansion of (a + b) n is given by
Tr+1 = nCr an-r br
Assuming that am occurs in the (r + 1) th term of the expansion (1 + a) m + n, we obtain
Tr+1 = m+nCr (1)m+n-r ar
Comparing the indices of a in am and in Tr + 1, we obtain
r = m
Therefore, the coefficient of am is
m+nCm = (m + n)! ÷ {m! × (m +n - m)!} = (m + n)! ÷ (m! × n!) …………… (1)
Assuming that an occurs in the (k + 1) th term of the expansion (1 + a) m+n, we obtain
Tk+1 = m+nCk (1)m+n-k ak = m+nCk ak
Comparing the indices of a in an and in Tk + 1, we obtain
k = n
Therefore, the coefficient of an is
m+nCn = (m + n)! ÷ {n! × (m +n - n)!} = (m + n)! ÷ (n! × m!) …………… (2)
Thus, from equation 1 and 2, it can be observed that the coefficients of am and an
in the expansion of (1 + a) m + n are equal.