Oh! Nice identity

Algebra Level 5

45 + 1 + 45 + 2 + 45 + 3 + + 45 + 2024 45 1 + 45 2 + 45 3 + + 45 2024 = ? \dfrac{\sqrt{45+\sqrt{1}}+\sqrt{45+\sqrt{2}}+\sqrt{45+\sqrt{3}}+\ldots+\sqrt{45+\sqrt{2024}}}{\sqrt{45-\sqrt{1}}+\sqrt{45-\sqrt{2}}+\sqrt{45-\sqrt{3}}+\ldots+\sqrt{45-\sqrt{2024}}} = \ ?

Submit your answer to three decimal places.


The answer is 2.4142.

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

5 solutions

Let S = 45 + 1 + 45 + 2 + 45 + 3 + + 45 + 2024 S=\sqrt{45+\sqrt{1}}+\sqrt{45+\sqrt{2}}+\sqrt{45+\sqrt{3}}+\ldots+\sqrt{45+\sqrt{2024}} and T = 45 1 + 45 2 + 45 3 + + 45 2024 T=\sqrt{45-\sqrt{1}}+\sqrt{45-\sqrt{2}}+\sqrt{45-\sqrt{3}}+\ldots+\sqrt{45-\sqrt{2024}}

We have:

S + T = i = 1 2024 ( 45 + 2025 i + 45 2025 i ) = i = 1 2024 ( 45 + 2025 i + 45 2025 i ) 2 = i = 1 2024 90 + 2 i = 2 S \displaystyle S+T=\sum_{i=1}^{2024}\left(\sqrt{45+\sqrt{2025-i}}+\sqrt{45-\sqrt{2025-i}}\right)\\ \qquad\;\;\displaystyle=\sum_{i=1}^{2024}\sqrt{\left(\sqrt{45+\sqrt{2025-i}}+\sqrt{45-\sqrt{2025-i}}\right)^2}\\ \qquad\;\;\displaystyle =\sum_{i=1}^{2024}\sqrt{90+2\sqrt{i}}=\sqrt{2}S

Thus, T = ( 2 1 ) S T=(\sqrt{2}-1)S or P = S T = 1 2 1 = 2 + 1 2.414 P=\dfrac{S}{T}=\dfrac{1}{\sqrt{2}-1}=\sqrt{2}+1\approx\boxed{2.414}

Moderator note:

What made you think of this relation? That seems to be the crux of the problem.

How did you come up with this problem?

@Khang Nguyen Thanh

You can prove a general theorem -

k = 1 a 2 1 a + k k = 1 a 2 1 a k = 2 + 1 \displaystyle \frac{\displaystyle \sum_{k=1}^{{a}^{2}-1}{\sqrt{a + \sqrt{k}}}}{\displaystyle \sum_{k=1}^{{a}^{2}-1}{\sqrt{a - \sqrt{k}}}} = \sqrt{2} +1 for any positive integer a ( > 1 ) a(>1) .

You can prove it using the same method as you did(great +1).

Also, you can prove it as I did by using the formula a + b = a + a 2 b 2 + a a 2 b 2 \displaystyle \sqrt{a + \sqrt{b}} = \sqrt{\frac{a + \sqrt{{a}^{2} - b}}{2}} + \sqrt{\frac{a - \sqrt{{a}^{2} - b}}{2}} which can be easily derived by squaring and observation(I think it's quite easy to prove and so why not just remember it forever?)

S = k = 1 a 2 1 a + a 2 k 2 + a a 2 k 2 k = 1 a 2 1 a k S = \displaystyle \frac{\displaystyle \sum_{k=1}^{{a}^{2}-1}{\sqrt{\frac{a + \sqrt{{a}^{2} - k}}{2}} + \sqrt{\frac{a - \sqrt{{a}^{2} - k}}{2}}}}{\displaystyle \sum_{k=1}^{{a}^{2}-1}{\sqrt{a - \sqrt{k}}}}

S = 1 2 ( k = 1 a 2 1 a + a 2 k 2 k = 1 a 2 1 a k + k = 1 a 2 1 a a 2 k 2 k = 1 a 2 1 a k ) S = \displaystyle \frac{1}{\sqrt{2}}\left(\frac{\displaystyle \sum_{k=1}^{{a}^{2}-1}{\sqrt{\frac{a + \sqrt{{a}^{2} - k}}{2}}}}{\displaystyle \sum_{k=1}^{{a}^{2}-1}{\sqrt{a - \sqrt{k}}}} + \displaystyle \frac{\displaystyle \sum_{k=1}^{{a}^{2}-1}{\sqrt{\frac{a - \sqrt{{a}^{2} - k}}{2}}}}{\displaystyle \sum_{k=1}^{{a}^{2}-1}{\sqrt{a - \sqrt{k}}}}\right)

Now, if we change order of summation to a 2 k {a}^{2} - k we get back our S in the numerator of the first fraction and the 2nd fraction becomes 1.

S = 1 2 ( S + 1 ) S = \displaystyle \frac{1}{\sqrt{2}} \left(S + 1\right)

S = 2 + 1 S = \sqrt{2} + 1

But I believe that your method is much better.

Kartik Sharma - 5 years, 9 months ago

Log in to reply

OH MY GAUSS! AWESOME SOLUTION!

Angel T. López - 5 years, 8 months ago

Thanks! Always prefer a general method than a lightning of aspiration.

林晓 耿 - 5 years, 6 months ago

I did same.

Dev Sharma - 5 years, 5 months ago

Log in to reply

Thus,Dev is a genius!

Rohit Udaiwal - 5 years, 5 months ago

Log in to reply

@Rohit Udaiwal No. I am not. But you are brilliant.

Dev Sharma - 5 years, 5 months ago

That is very interesting!

Calvin Lin Staff - 5 years, 10 months ago

Wow! I learnt a new method today. You just nailed it.

Satyajit Mohanty - 5 years, 10 months ago

Wow. Such maths. Cannot handle.

Trung Đặng Đoàn Đức - 5 years, 8 months ago

@Khang Nguyen Thanh Awesome solution....Beyond my imagination

Ankit Kumar Jain - 5 years ago

Wow that looks very simple for him. Interesting =)

Keil Cerbito - 5 years, 10 months ago

Log in to reply

A few days back, I came across this solution on some problem at a random website and I liked it.. Super easy

Hrishik Mukherjee - 5 years, 9 months ago

Wow! I would never think that simple! I did much complicated.

Adam Phúc Nguyễn - 5 years, 10 months ago

How did you find out sum of √[90+(2√i)]

Sameer Marathe - 5 years, 10 months ago

Log in to reply

i = 1 2024 90 + 2 i = 2 × [ i = 1 2024 45 + i ] = 2 S \displaystyle \sum\limits_{i=1}^{2024} \sqrt{90+2\sqrt{i}} = \sqrt{2}\times\left[\sum\limits_{i=1}^{2024} \sqrt{45+\sqrt{i}}\right] = \sqrt{2}S

Kishore S. Shenoy - 5 years, 9 months ago

Same Method

Vraj Mistry - 5 years, 9 months ago

Flawless Solution!!! \textit{Flawless Solution!!!}

Cleres Cupertino - 5 years, 9 months ago

elelegant solution

Refaat M. Sayed - 5 years, 9 months ago

Sick Problem!

Alan Yan - 5 years, 9 months ago

WOWOWOWOWOW!!! Really astonished!

Kishore S. Shenoy - 5 years, 9 months ago

Awesome solution

Nikkil V - 4 years, 5 months ago
Menachem Avinoam
Nov 18, 2015

I got my answer from wolframapha with:

(summation sqrt(45+sqrt(i)),i=1 to 2024)/(summation sqrt(45-sqrt(i)),i=1 to 2024)

What did you learn?

Yasharyan Gaikwad - 5 years, 3 months ago

Wolfram alpha=cheating

Armain Labeeb - 4 years, 11 months ago

Nice man! I solved it using python in a similar way. Nothing wrong with using the tools we have available.

Faraz Arastu - 4 years, 10 months ago

Apha or alpha ? :)

Harout G. Vartanian - 4 years, 5 months ago

I also used Wolfram Alpha.

Niranjan Khanderia - 4 years, 3 months ago

We can start by thinking: what would happen if we subtracted 45 x \sqrt{45-\sqrt{x}} from 45 + x \sqrt{45+\sqrt{x}} ? Knowing what happens might be useful in some way. Let: y = 45 + x 45 x y=\sqrt{45+\sqrt{x}}-\sqrt{45-\sqrt{x}} , so 45 x + y = 45 + x \sqrt{45-\sqrt{x}}+y=\sqrt{45+\sqrt{x}} .

Then: y 2 = ( 45 + x ) + ( 45 x ) + 2 ( 45 + x ) ( 45 x ) y^2=(45+\sqrt{x})+(45 -\sqrt{x})+2\sqrt{(45+\sqrt{x})(45-\sqrt{x})} .

Simplifying: y 2 = 90 2 2025 x y^2=90-2\sqrt{2025-x} , so y = 90 2 2025 x y=\sqrt{90-2\sqrt{2025-x}} .

That means 45 x + 90 2 2025 x = 45 + x \sqrt{45-\sqrt{x}}+\sqrt{90-2\sqrt{2025-x}}=\sqrt{45+\sqrt{x}} .

We can already see that it is getting useful. Now, back to the problem. We can use our identity here:

45 + 1 + 45 + 2 + 45 + 3 + + 45 + 2024 45 1 + 45 2 + 45 3 + + 45 2024 \frac{\sqrt{45+\sqrt{1}}+\sqrt{45+\sqrt{2}}+\sqrt{45+\sqrt{3}}+\ldots+\sqrt{45+\sqrt{2024}}}{\sqrt{45-\sqrt{1}}+\sqrt{45-\sqrt{2}}+\sqrt{45-\sqrt{3}}+\ldots+\sqrt{45-\sqrt{2024}}} = x = 1 2024 45 + x x = 1 2024 45 x = \frac{\displaystyle \sum_{x=1}^{2024} \sqrt{45+\sqrt{x}}}{\displaystyle \sum_{x=1}^{2024} \sqrt{45-\sqrt{x}}}

= x = 1 2024 45 x + x = 1 2024 90 2 2025 x x = 1 2024 45 x = \frac{\displaystyle \sum_{x=1}^{2024} \sqrt{45-\sqrt{x}}+\displaystyle \sum_{x=1}^{2024} \sqrt{90-2\sqrt{2025-x}}}{\displaystyle \sum_{x=1}^{2024} \sqrt{45-\sqrt{x}}} = 1 + x = 1 2024 90 2 2025 x x = 1 2024 45 x = 1 + \frac{\displaystyle \sum_{x=1}^{2024} \sqrt{90-2\sqrt{2025-x}}}{\displaystyle \sum_{x=1}^{2024} \sqrt{45-\sqrt{x}}} .

The numerator can be reordered and the sum can be 'reversed':

= 1 + x = 1 2024 90 2 x x = 1 2024 45 x = 1 + \frac{\displaystyle \sum_{x=1}^{2024} \sqrt{90-2\sqrt{x}}}{\displaystyle \sum_{x=1}^{2024} \sqrt{45-\sqrt{x}}} . Now, it's getting interesting:

= 1 + x = 1 2024 2 45 x x = 1 2024 45 x = 1 + \frac{\displaystyle \sum_{x=1}^{2024} \sqrt{2}\sqrt{45-\sqrt{x}}}{\displaystyle \sum_{x=1}^{2024} \sqrt{45-\sqrt{x}}} . Cancelling:

= 1 + 2 2.414 = 1 + \sqrt{2} \approx\ \boxed{2.414}

Ashutosh Kumar
May 26, 2016

Woow too good

Deep Joshi
Aug 20, 2015

write this code in java you will get the answer : double d=0.0,e=0.0; for(int i=1;i<=2024;i++) { d = d + (Math.sqrt(45 + Math.sqrt(i))); e = e + (Math.sqrt(45 - Math.sqrt(i))); } System.out.println("Final ans is :"+d/e);

If you pretend to use some code is exactly the same that using calculator.

Matías Bruna - 5 years, 9 months ago

Log in to reply

Thnx for commenting. Now I will not use that.

Deep Joshi - 5 years, 9 months ago

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...