Can it be summed up?

Algebra Level 5

n = 1 9800 1 n + n 2 1 \large \sum_{n = 1}^{9800} \dfrac{1}{\sqrt{n + \sqrt{n^2 - 1}}}

If the sum above can be expressed as p + q r p + q \sqrt{r} , where p , q p, q and r r are positive integers and r r is not divisible by the square of any prime, determine p + q + r p + q + r .


This problem is adopted form a mock test of a renowned contest.


The answer is 121.

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1 solution

Aareyan Manzoor
Nov 14, 2015

i offer you this a b = c 2 d a + b 2 a b = c 2 d |\sqrt{a}-\sqrt{b}|=\sqrt{c-2\sqrt{d}}\Longrightarrow a+b-2\sqrt{ab}=c-2\sqrt{d} a + b = c , d = a b a+b=c, d=ab using this : 1 n + n 2 1 = n n 2 1 n + n 2 1 n n 2 1 = n n 2 1 \dfrac{1}{\sqrt{n+\sqrt{n^2-1}}}=\dfrac{\sqrt{n-\sqrt{n^2-1}}}{\sqrt{n+\sqrt{n^2-1}}*\sqrt{n-\sqrt{n^2-1}}}=\sqrt{n-\sqrt{n^2-1}} n n 2 1 = 2 n 2 n 2 1 2 = n + 1 n 1 2 \sqrt{n-\sqrt{n^2-1}}=\dfrac{\sqrt{2n-2\sqrt{n^2-1}}}{\sqrt{2}}=\dfrac{\sqrt{n+1}-\sqrt{n-1}}{\sqrt{2}} we put this in the summation and 1 2 n = 1 9800 ( n + 1 n 1 ) \dfrac{1}{\sqrt{2}}\sum_{n=1}^{9800} (\sqrt{n+1}-\sqrt{n-1}) it easily telescopes as 2 2 ( 1 1 2 1 + 9799 + 1 + 9800 + 1 ) = 2 2 ( 98 + 70 2 ) = 70 + 49 2 \dfrac{\sqrt{2}}{2}(-\sqrt{1-1}-\sqrt{2-1}+\sqrt{9799+1}+\sqrt{9800+1})=\dfrac{\sqrt{2}}{2}(98+70\sqrt{2})=70+49\sqrt{2} 70 + 49 + 2 = 121 70+49+2=\boxed{121}

Wow.. Great

Anil Sharma - 5 years, 7 months ago

Great!!! I think everyone did it in this way... :p

Ahmed Arup Shihab - 5 years, 7 months ago

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cannot think of any other way that does not include wolfram alpha.

Aareyan Manzoor - 5 years, 7 months ago

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Almost arrived to the solution ! But unfortunately didn't add √9801 at the last !!!!

Aditya Sky - 5 years, 7 months ago

I also did it basically the same way! :D

James Wilson - 2 years, 5 months ago

Nice solution!!

Yao Feng Ooi - 5 years, 6 months ago

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