Difference Of Nested Radicals

Calculus Level 1

S 1 = n + n n + n S 2 = n n + n n + \begin{aligned} S_{1}&=&\sqrt{n+\sqrt{n-\sqrt{n+\sqrt{n-\cdots}}}} \\ S_{2}&=&\sqrt{n-\sqrt{n+\sqrt{n-\sqrt{n+\cdots}}}} \end{aligned}

When both infinitely nested radicals are defined, what is S 1 S 2 S_1 -S_2 ?


The answer is 1.

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3 solutions

Matías Bruna
Feb 7, 2016

S 1 = n + S 2 S_{1}=\sqrt{n+S_{2}} and S 2 = n S 1 S_{2}=\sqrt{n-S_{1}} Squaring both equations: S 1 2 = n + S 2 {S_{1}}^{2}=n+S_{2} and S 2 2 = n S 1 {S_{2}}^{2}=n-S_{1}

Subtract them: S 1 2 S 2 2 = S 2 + S 1 ( S 1 + S 2 ) ( S 1 S 2 ) = S 1 + S 2 {S_{1}}^{2}-{S_{2}}^{2}=S_{2}+S_{1} \rightarrow (S_{1}+S_{2})(S_{1}-S_{2})=S_{1}+S_{2}

Since S 1 + S 2 0 S 1 S 2 = 1 S_{1}+S_{2} \neq 0 \rightarrow \boxed{S_{1}-S_{2}=1}

For small n n ( n = 1.1 n=1.1 , say) these nested radicals are not even defined.

Otto Bretscher - 5 years, 4 months ago

ya for what if we have n 254

Biswajit Barik - 4 years ago
William Isoroku
Feb 11, 2016

Basically:

S 1 2 = n + S 2 S_1^2=n+S_2

S 2 2 = n S 1 S_2^2=n-S_1

Subtracting the 2 equations give:

S 1 2 S 2 2 = S 1 + S 2 S_1^2-S_2^2=S_1+S_2

Factor the LHS as difference of 2 squares and dividing will yield the answer.

Use quadratic equation to prove

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