Nested Radicals of Geometric Sequences

Algebra Level 4

Design a formula to calculate nested radicals of the form: 1 n n 2 n 3 m m m m \large \sqrt[m]{1\sqrt[m]{n\sqrt[m]{n^2\sqrt[m]{n^3\cdots}}}}

where n n is a postive real number and m m is a positive integer.

Using the formula you got from previous step, the result of: 1 3 9 27 5 5 5 5 \large \sqrt[5]{1\sqrt[5]{3\sqrt[5]{9\sqrt[5]{27\cdots}}}} will be in the form of y x \sqrt[x]{y} , y y is prime. What is x + y x+y ?


The answer is 19.

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2 solutions

Rishabh Jain
Feb 18, 2016

Q = 1 n n 2 n 3 m m m m \large \mathfrak{Q}=\sqrt[m]{1\sqrt[m]{n\sqrt[m]{n^2\sqrt[m]{n^3\cdots}}}} = ( n 1 m 2 ) ( n 2 ( 1 m 3 ) ) ( n 3 ( 1 m 4 ) ) \large =(n^{\frac{1}{m^2}})(n^{2(\frac{1}{m^3})})(n^{3(\frac{1}{m^4})})\cdots = n 1 m 2 + 2 m 3 + 3 m 4 + 4 m 5 \large =n^{\color{#20A900}{\frac{1}{m^2}+\frac{2}{m^3}+\frac{3}{m^4}+\frac{4}{m^5}\cdots}} Let S = 1 m 2 + 2 m 3 + 3 m 4 + 4 m 5 ( A G P ) \large\mathfrak{S}=\frac{1}{m^2}+\frac{2}{m^3}+\frac{3}{m^4}+\frac{4}{m^5}\cdots (\small{\color{#302B94}{AGP}})~~ S m = 1 m 3 + 2 m 4 + 3 m 5 + \dfrac{\mathfrak S}{m}=~~~~~~~~~~~~\frac{1}{m^3}+\frac{2}{m^4}+\frac{3}{m^5}+\cdots~~~~~~~~~ Subtracting we get S ( 1 1 m ) = 1 m 2 + 1 m 3 + 1 m 4 ( Infinite GP ) \mathfrak S(1-\frac{1}{m})=\color{#D61F06}{\frac{1}{m^2}+\frac{1}{m^3}+\frac{1}{m^4}\cdots} (\small{\color{#302B94}{\text{Infinite GP}}})
(For positive integer 'm' this is a Infinite GP) S = 1 ( m 1 ) 2 \large \Rightarrow \mathfrak S=\frac{1}{(m-1)^2} Q = n 1 ( m 1 ) 2 \large\Rightarrow \mathfrak Q=n^{\frac{1}{(m-1)^2}} For n=3, m=5 we get Q = 3 1 16 = 1 3 9 27 5 5 5 5 \large \mathfrak Q=3^{\frac{1}{16}}= \sqrt[5]{1\sqrt[5]{3\sqrt[5]{9\sqrt[5]{27\cdots}}}} 16 + 3 = 19 \huge \therefore~16+3=\boxed{\color{#007fff}{19}}

Otto Bretscher
Feb 18, 2016

3 1 / 25 9 1 / 125 2 7 1 / 625 . . . = 3 1 / 25 + 2 / 125 + 3 / 625 + . . . = 3 1 / 16 = 3 16 3^{1/25}*9^{1/125}*27^{1/625}...=3^{1/25+2/125+3/625+...}=3^{1/16}=\sqrt[16]{3} so that the answer is 19 \boxed{19}

The general case is n 1 / m 2 + 2 / m 3 + 3 / m 4 + . . . = n 1 / ( m 1 ) 2 n^{1/m^2+2/m^3+3/m^4+...}=n^{1/(m-1)^2}

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