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Question
In the binomial expansion of (a + b)n, if the coefficients of the 4th and 13th terms are equal then, find n
Solution
In (a + b)n general term is tr + 1 = nCr an – rbr
So, t4 = t3 + 1
= nC3
= nC12
⇒ n = 12 + 3
= 15
We are given that their coefficients are equal ⇒ nC3 = nC12
⇒ n = 12 + 3
= 15
[nCx = nCy ⇒ x = y (or) x + y = n]
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