4 5 − 1 + 4 5 − 2 + 4 5 − 3 + … + 4 5 − 2 0 2 4 4 5 + 1 + 4 5 + 2 + 4 5 + 3 + … + 4 5 + 2 0 2 4 = ?
Submit your answer to three decimal places.
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What made you think of this relation? That seems to be the crux of the problem.
How did you come up with this problem?
You can prove a general theorem -
k = 1 ∑ a 2 − 1 a − k k = 1 ∑ a 2 − 1 a + k = 2 + 1 for any positive integer 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 = 2 a + a 2 − b + 2 a − a 2 − b 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 − k k = 1 ∑ a 2 − 1 2 a + a 2 − k + 2 a − a 2 − k
S = 2 1 ⎝ ⎜ ⎜ ⎜ ⎜ ⎜ ⎛ k = 1 ∑ a 2 − 1 a − k k = 1 ∑ a 2 − 1 2 a + a 2 − k + k = 1 ∑ a 2 − 1 a − k k = 1 ∑ a 2 − 1 2 a − a 2 − k ⎠ ⎟ ⎟ ⎟ ⎟ ⎟ ⎞
Now, if we change order of summation to a 2 − k we get back our S in the numerator of the first fraction and the 2nd fraction becomes 1.
S = 2 1 ( S + 1 )
S = 2 + 1
But I believe that your method is much better.
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OH MY GAUSS! AWESOME SOLUTION!
Thanks! Always prefer a general method than a lightning of aspiration.
I did same.
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Thus,Dev is a genius!
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@Rohit Udaiwal – No. I am not. But you are brilliant.
That is very interesting!
Wow! I learnt a new method today. You just nailed it.
Wow. Such maths. Cannot handle.
@Khang Nguyen Thanh Awesome solution....Beyond my imagination
Wow that looks very simple for him. Interesting =)
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A few days back, I came across this solution on some problem at a random website and I liked it.. Super easy
Wow! I would never think that simple! I did much complicated.
How did you find out sum of √[90+(2√i)]
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i = 1 ∑ 2 0 2 4 9 0 + 2 i = 2 × [ i = 1 ∑ 2 0 2 4 4 5 + i ] = 2 S
Same Method
Flawless Solution!!!
elelegant solution
Sick Problem!
WOWOWOWOWOW!!! Really astonished!
Awesome solution
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?
Wolfram alpha=cheating
Nice man! I solved it using python in a similar way. Nothing wrong with using the tools we have available.
Apha or alpha ? :)
I also used Wolfram Alpha.
We can start by thinking: what would happen if we subtracted 4 5 − x from 4 5 + x ? Knowing what happens might be useful in some way. Let: y = 4 5 + x − 4 5 − x , so 4 5 − x + y = 4 5 + x .
Then: y 2 = ( 4 5 + x ) + ( 4 5 − x ) + 2 ( 4 5 + x ) ( 4 5 − x ) .
Simplifying: y 2 = 9 0 − 2 2 0 2 5 − x , so y = 9 0 − 2 2 0 2 5 − x .
That means 4 5 − x + 9 0 − 2 2 0 2 5 − x = 4 5 + x .
We can already see that it is getting useful. Now, back to the problem. We can use our identity here:
4 5 − 1 + 4 5 − 2 + 4 5 − 3 + … + 4 5 − 2 0 2 4 4 5 + 1 + 4 5 + 2 + 4 5 + 3 + … + 4 5 + 2 0 2 4 = x = 1 ∑ 2 0 2 4 4 5 − x x = 1 ∑ 2 0 2 4 4 5 + x
= x = 1 ∑ 2 0 2 4 4 5 − x x = 1 ∑ 2 0 2 4 4 5 − x + x = 1 ∑ 2 0 2 4 9 0 − 2 2 0 2 5 − x = 1 + x = 1 ∑ 2 0 2 4 4 5 − x x = 1 ∑ 2 0 2 4 9 0 − 2 2 0 2 5 − x .
The numerator can be reordered and the sum can be 'reversed':
= 1 + x = 1 ∑ 2 0 2 4 4 5 − x x = 1 ∑ 2 0 2 4 9 0 − 2 x . Now, it's getting interesting:
= 1 + x = 1 ∑ 2 0 2 4 4 5 − x x = 1 ∑ 2 0 2 4 2 4 5 − x . Cancelling:
= 1 + 2 ≈ 2 . 4 1 4
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.
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Let S = 4 5 + 1 + 4 5 + 2 + 4 5 + 3 + … + 4 5 + 2 0 2 4 and T = 4 5 − 1 + 4 5 − 2 + 4 5 − 3 + … + 4 5 − 2 0 2 4
We have:
S + T = i = 1 ∑ 2 0 2 4 ( 4 5 + 2 0 2 5 − i + 4 5 − 2 0 2 5 − i ) = i = 1 ∑ 2 0 2 4 ( 4 5 + 2 0 2 5 − i + 4 5 − 2 0 2 5 − i ) 2 = i = 1 ∑ 2 0 2 4 9 0 + 2 i = 2 S
Thus, T = ( 2 − 1 ) S or P = T S = 2 − 1 1 = 2 + 1 ≈ 2 . 4 1 4