Rationalizing sixth roots?

Algebra Level 4

Find the integer which is closest to the value of 1 5 6 + 1 6 5 6 1 6 \dfrac { 1 }{ \sqrt [ 6 ]{ { 5 }^{ 6 }+1 } -\sqrt [ 6 ]{ { 5 }^{ 6 }-1 } } .


The answer is 9375.

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1 solution

Chew-Seong Cheong
Nov 20, 2016

Relevant wiki: Taylor Series Approximation

1 5 6 + 1 6 5 6 1 6 = 1 5 1 + 1 5 6 6 5 1 + 1 5 6 6 By Taylor’s series expansion = 1 5 ( 1 + 1 6 1 5 6 5 72 1 5 12 + . . . ) 5 ( 1 1 6 1 5 6 5 72 1 5 12 . . . ) 1 5 ( 2 6 1 5 6 ) = 3 × 5 5 = 9375 \begin{aligned} \frac 1{\sqrt[6]{5^6+1}-\sqrt[6]{5^6-1}} & = \frac 1{5{\color{#3D99F6}\sqrt[6]{1+\frac 1{5^6}}}-5{\color{#3D99F6}\sqrt[6]{1+\frac 1{5^6}}}} \quad \quad \small {\color{#3D99F6}\text{By Taylor's series expansion}} \\ & = \frac 1{5 {\color{#3D99F6} \left(1 + \frac 16 \cdot \frac 1{5^6} - \frac 5{72} \cdot \frac 1{5^{12}} + ... \right)} - 5 {\color{#3D99F6} \left(1 - \frac 16 \cdot \frac 1{5^6} - \frac 5{72} \cdot \frac 1{5^{12}} - ... \right)}} \\ & \approx \frac 1{5\left(\frac 26 \cdot \frac 1{5^6} \right)} = 3\times 5^5 = \boxed{9375} \end{aligned}

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