frac{1}{4}-frac{5}{4 cdot 8}+frac{5 cdot 7}{4 | vedclass

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(C) Let $S = \frac{1}{4} - \frac{5}{4 \cdot 8} + \frac{5 \cdot 7}{4 \cdot 8 \cdot 12} - \ldots$ Multiply and divide by $3$: $S = \frac{1}{3} \left( \f

Vedclass · Accepted Answer

$\frac{3 \sqrt{3}-2 \sqrt{2}}{9 \sqrt{3}}$

Vedclass · Answer

(C) Let $S = \frac{1}{4} - \frac{5}{4 \cdot 8} + \frac{5 \cdot 7}{4 \cdot 8 \cdot 12} - \ldots$ Multiply and divide by $3$: $S = \frac{1}{3} \left( \frac{3}{4} - \frac{3 \cdot 5}{4 \cdot 8} + \frac{3 \cdot 5 \cdot 7}{4 \cdot 8 \cdot 12} - \ldots \right)$ We know the binomial expansion $(1+x)^{-n} = 1 - nx + \frac{n(n+1)}{2!}x^2 - \frac{n(n+1)(n+2)}{3!}x^3 + \ldots$ Comparing the series inside the bracket with the expansion,we set $n = \frac{3}{2}$ and $x = \frac{1}{2}$. The series $1 - \frac{3}{2}(\frac{1}{2}) + \frac{\frac{3}{2} \cdot \frac{5}{2}}{2!}(\frac{1}{2})^2 - \ldots$ corresponds to $(1 + \frac{1}{2})^{-3/2}$. Since our series starts from the second term,we have: $\frac{3}{4} - \frac{3 \cdot 5}{4 \cdot 8} + \ldots = 1 - (1 + \frac{1}{2})^{-3/2} = 1 - (\frac{3}{2})^{-3/2} = 1 - (\frac{2}{3})^{3/2} = 1 - \frac{2\sqrt{2}}{3\sqrt{3}} = \frac{3\sqrt{3} - 2\sqrt{2}}{3\sqrt{3}}$. Substituting this into the expression for $S$: $S = \frac{1}{3} \left( \frac{3\sqrt{3} - 2\sqrt{2}}{3\sqrt{3}} \right) = \frac{3\sqrt{3} - 2\sqrt{2}}{9\sqrt{3}}$.

$\frac{1}{4}-\frac{5}{4 \cdot 8}+\frac{5 \cdot 7}{4 \cdot 8 \cdot 12}-\ldots=$

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