Law of Exponent and Telescopic Sum

Algebra Level 3

32 3 32 3 4 32 4 5 32 9 10 = ? \large\sqrt[3]{\sqrt{32}}\sqrt[4]{\sqrt[3]{32}}\sqrt[5]{\sqrt[4]{32}} \cdots \sqrt[10]{\sqrt[9]{32}} = \, ?


The answer is 4.

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1 solution

Snehal Shekatkar
Jan 16, 2015

By the rule of indices, we know that ( x m ) n = x m n (x^{m})^{n}=x^{mn} . Hence the Given entity can be written in the following form: A = 3 2 1 2 × 3 3 2 1 3 × 4 3 2 1 4 × 5 . . . 3 2 1 9 × 10 A = 32^{\frac{1}{2\times 3}}32^{\frac{1}{3\times 4}}32^{\frac{1}{4\times 5}}...32^{\frac{1}{9\times 10}} Again using the fact that x m x n = x m + n x^{m}x^{n}=x^{m+n} , this can be reduced to following form: A = 3 2 x A = 32^{x} where x = 1 2 × 3 + 1 3 × 4 + 1 4 × 5 + . . . + 1 9 × 10 x = \frac{1}{2\times 3}+\frac{1}{3\times 4}+\frac{1}{4\times 5}+...+\frac{1}{9\times 10}

This could be turned into a famous Telescoping sum like this: x = ( 1 2 1 3 ) + ( 1 3 1 4 ) + ( 1 4 1 5 ) + . . . + ( 1 9 1 10 ) x = \left(\frac{1}{2}-\frac{1}{3}\right)+\left(\frac{1}{3}-\frac{1}{4}\right)+\left(\frac{1}{4}-\frac{1}{5}\right)+...+\left(\frac{1}{9}-\frac{1}{10}\right) Cancelling intermediate terms, we get: x = 1 2 1 10 = 2 5 x=\frac{1}{2}-\frac{1}{10}=\frac{2}{5} Hence we have: A = 2 5 × 2 5 = 4 A = 2^{5\times \frac{2}{5}}=\boxed{4}

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