Question:

If $x$ is so small that $x^3$ and higher powers of $x$ may be neglected, then $\frac{\left(1+x\right)^{\frac{3}{2}} - \left(1+ \frac{1}{2}x\right)^{3}}{\left(1-x\right)^{\frac{1}{2}}} $ may be approximated as

Updated On: Jul 5, 2022
  • $ 1 - \frac{3}{8} x^2$
  • $ 3x + \frac{3}{8} x^2$
  • $ - \frac{3}{8} x^2$
  • $ \frac{x}{2} - \frac{3}{8} x^2$
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The Correct Option is C

Solution and Explanation

$\because \, x^3 $ and higher powers of x may be neglected $ \therefore \frac{\left(1+x\right) \frac{3}{2} -\left(1+\frac{x}{2}\right)^{3}}{\left(1-x^{\frac{1}{2}}\right)} $ $=\left(1-x\right)^{\frac{-1}{2}} \left[\left(1+\frac{3}{2} x + \frac{\frac{3}{2}. \frac{1}{2}}{2!} x^{2}\right) - \left(1+ \frac{3x}{2} + \frac{3.2}{2!} \frac{x^{2}}{4}\right)\right] $ $= \left[1+ \frac{x}{2} + \frac{\frac{1}{2} . \frac{3}{2}}{2!}x^{2}\right] \left[ \frac{-3}{8} x^{2}\right] = \frac{-3}{8} x^{2} $ (as $x^3$ and higher powers of x can be neglected)
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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.