Question:

The term independent of $x$ in the expansion of $(x- \frac {3} {x^2})^{18}$

Updated On: Jul 7, 2022
  • $^{18}C_6 $
  • $^{18}C_6 \, 3^6 $
  • $^{18}C_{12} \, {3}^{-6} $
  • $3^6$
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The Correct Option is B

Solution and Explanation

$T_{r+1} = \,^{18}c_{r}x^{18-r} \left(-\frac{3}{x^{2}}\right)^{r}$ $= \, ^{18}c_{r}x^{18-r-2r}\left(-3\right)^{r}$ $\therefore 18 - 3r = 0$ $\Rightarrow r = 6$ $\therefore$ reqd. term $=\, ^{18}c_{6}\left(-3\right)^{6} = \, ^{18}c_{6}\left(3\right)^{6}$.
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Concepts Used:

Binomial Theorem

The binomial theorem formula is used in the expansion of any power of a binomial in the form of a series. The binomial theorem formula is 

Properties of Binomial Theorem

  • The number of coefficients in the binomial expansion of (x + y)n is equal to (n + 1).
  • There are (n+1) terms in the expansion of (x+y)n.
  • The first and the last terms are xn and yn respectively.
  • From the beginning of the expansion, the powers of x, decrease from n up to 0, and the powers of a, increase from 0 up to n.
  • The binomial coefficients in the expansion are arranged in an array, which is called Pascal's triangle. This pattern developed is summed up by the binomial theorem formula.