NCERT Solutions
Class 11 Maths
Binomial Theorem

Ex.Misc.Q.2
Find a if the coefficients of x2 and x3 in the expansion of (3 + ax)9 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 x2 occurs in the (r + 1) th term in the expansion of (3 + ax)9, we obtain
Tr+1 = 9Cr (3)9 - r (ax)r = 9Cr ar xr
Comparing the indices of x in x2 and in Tr + 1, we obtain r = 2
Thus, the coefficient of x2 is
9C2 39-2 a2 = {9! ÷ (2! × 7!)} × 37 a2 = 36 × 37 a2
Assuming that x3 occurs in the (k + 1) th term in the expansion of (3 + ax)9, we obtain
Tk+1 = 9Ck (3)9 - r (ax)k = 9Cr (3)9 - k ak xk
Comparing the indices of x in x3 and in Tk+ 1, we obtain k = 3
Thus, the coefficient of x3 is
9C3 39-3 a3 = {9! ÷ (3! × 6!)} × 36 a3 = 84 × 36 a3
It is given that the coefficients of x2 and x3 are the same.
=> 84 × 36 a3 = 36 × 37 a2
=> 84a = 36 × 3
=> a =
=> a =
Thus, the required value of a is .