Question:

The coefficient of $x^n$ in the expansion of $\log_a (1 + x)$ is

Updated On: Jun 18, 2022
  • $\frac{(-1)^{n - 1}}{n}$
  • $\frac{(-1)^{n - 1}}{n} \log_a \, e $
  • $\frac{(-1)^{n - 1}}{n} \log_e \, a $
  • $\frac{(-1)^{n }}{n} \log_a \, e $
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The Correct Option is B

Solution and Explanation

$\log_{a} \left(1+x\right) = \log_{e} \left(1+x\right)\log_{a}e$
$ = \log_{a}e \left[\displaystyle\sum_{n=1}^{\infty}\left(-1\right)^{n-1} \frac{X^{n}}{n}\right]$
So, the coefficient of $x^n$ in $\log_a ( 1 + x)$ is
$ \frac{\left(-1\right)^{n- 1}}{n} \log_{a}e $
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Concepts Used:

Binomial Expansion Formula

The binomial expansion formula involves binomial coefficients which are of the form 

(n/k)(or) nCk and it is calculated using the formula, nCk =n! / [(n - k)! k!]. The binomial expansion formula is also known as the binomial theorem. Here are the binomial expansion formulas.

This binomial expansion formula gives the expansion of (x + y)n where 'n' is a natural number. The expansion of (x + y)n has (n + 1) terms. This formula says:

We have (x + y)n = nC0 xn + nC1 xn-1 . y + nC2 xn-2 . y2 + … + nCn yn

General Term = Tr+1 = nCr xn-r . yr

  • General Term in (1 + x)n is nCr xr
  • In the binomial expansion of (x + y)n , the rth term from end is (n – r + 2)th .