NCERT Solutions
Class 11 Maths
Binomial Theorem

Ex.Misc Q.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.
Given that the first three terms of the expansion are 729, 7290 and 30375, respectively.
Now T1 = nC0 × an-0 × b0 = 729
=> an = 729 ................ (1)
T2 = nC1 × an-1 × b1 = 7290
=> n × an-1 × b = 7290 .......(2)
T3 = nC2 × an-2 × b2 = 30375
=> {n (n - 1) ÷ 2} × an-2 × b2 = 30375 .......(3)
Now equation2/equation1
n × an-1 × (b ÷ an) = 7290 ÷ 729
=> n × = 10 .......(4)
Now equation (3) ÷ equation (2)
{n (n - 1) ÷ 2} × an-2 × b2 ÷ n × an-1 × b = 30375 ÷ 7290
=> b (n - 1) ÷ 2a = 30375 ÷ 7290
=> b (n - 1) ÷ a = (30375 × 2) ÷ 7290
=> n -
= 60750 ÷ 7290
=> 10 - = 6075 ÷ 729
(60750 and 7290 is divided by 10)
=> 10 - =
(6075 and 729 is divided by 243)
=> 10 - =
=> (30 - 25) ÷ 3 =
=> = ba
=> =
................. (5)
Put this value in equation (4), we get
n × = 10
=> 5n = 30
=> n =
=> n = 6
Now put this value in equation (1), we get
a6 = 729
=> a6 = 36
=> a = 3
Now from equation (5), we get
=
=> b = 5
So, the values of a, b and n are 3, 5 and 6 respectively.