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Algebra Level 1

1 + 3 + 5 + + 53 3 = ? \large \color{#D61F06}{\sqrt[3]{1+3+5+ \ldots + 53}= \ ?}


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The answer is 9.

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8 solutions

Abhishek Sharma
May 12, 2015

1 + 3 + 5 + . . . . . + 53 3 \sqrt[3]{1+3+5+.....+53}

The difference between two consecutive terms is same therefore it an A.P. with common difference 2 2 .

Let 53 53 be the n t h nth term of the sequence.

53 = 1 + ( n 1 ) 2 53=1+(n-1)2

n = 27 n=27

The sum of the sequence is 27 2 ( 1 + 53 ) \frac{27}{2} (1+53) or 27 2 {27}^{2} .

We are required to find 27 2 3 \sqrt[3]{{27}^{2}} which is,

9 \boxed{9}

Moderator note:

Good. Can you find the smallest odd integer n > 53 n> 53 such that 1 + 3 + 5 + + ( n 2 ) + n 3 \sqrt[3]{1 + 3 + 5 + \ldots + (n-2) + n } is an integer?

In response to Challenge Master: From the solution we can conclude that n n must be a perfect cube to obtain an integer from the expression. The next cube after 27 27 is 64 64 . a n = 1 + ( 64 1 ) 2 = 1 + 63 ( 2 ) = 127 a_{ n }=1+(64-1)2=1+63(2)=\boxed{127} .

Rohit Ner - 6 years, 1 month ago

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Yeah, you are right. :)

Nihar Mahajan - 6 years, 1 month ago

Meh. Nice one! :)

Mehul Arora - 6 years, 1 month ago

Good point rohit:-)

prakhar mishra - 5 years, 11 months ago

Note first that, in general for any integer n 1 , n \ge 1,

k = 1 n ( 2 k 1 ) = 2 k = 1 n k k = 1 n 1 = 2 n ( n + 1 ) 2 n = n ( n + 1 ) n = n 2 . \displaystyle\sum_{k=1}^{n} (2k - 1) = 2\sum_{k=1}^{n} k - \sum_{k=1}^{n} 1 = 2*\dfrac{n(n + 1)}{2} - n = n(n + 1) - n = n^{2}.

In this case, since 53 = 2 27 1 , 53 = 2*27 - 1, we see that the given expression is

( 2 7 2 ) 1 3 = ( 2 7 1 3 ) 2 = 3 2 = 9 . (27^{2})^{\frac{1}{3}} = (27^{\frac{1}{3}})^{2} = 3^{2} = \boxed{9}.

Vishnu Bhagyanath
May 15, 2015

Sum of n odd numbers is n 2 n^2 . 1 + 3 + 5 + + 53 3 \sqrt[3]{1+3+5+ \dots + 53} 2 7 2 3 \sqrt[3]{27^2} 9 \boxed{9}

Moderator note:

It would be better to explain why the sum of the first n n odd numbers is equals to n 2 n^2 . Can you prove this statement? Hint: Arithmetic progression sum.

( 1 + 53 ) + ( 3 + 51 ) + ( 5 + 49 ) + + ( 25 + 29 ) + ( 27 ) ( 1 + 53) + (3 + 51) + (5 +49) + \ldots + (25 + 29) + (27)

Can you take it from here?

  • Sum of the first n odd numbers =[n ×( the last number + the first number)]/ 2
  • =[n×{2(n-1)+1 +1}] /2
  • =(n×2n)/2
  • =n²

Hetav Panchani - 5 years, 11 months ago
Tasnim Rawat
May 13, 2015

n= (53+1)/2 = 27 Sn = n/2(a +L) =27/2(1+53) =27(27) =9 ^3 according to the question the whole summation is under the power 1/3 so that 9 ^(3 *1/3) = 9

Girish Km
Jun 1, 2015

When numbers in arithmetic progression are added it can also be represented as the number of terms multiplied by the average of the smallest and largest number For ex:1+4+7=4 x 3 So this expression can be represented as 27 x 27

Nicholas Spinner
May 28, 2015

Here is the simple common sense solution:

For the sequence, 1+3+5+...+53.

  • Number of terms of 1 = 1
  • Number of terms of 3,5,...,53 = Number of terms of 2,4,...,52 = 52/2 = 26
  • Total Number of Terms = 1 + 26 = 27

  • Average Term Value = (1+53)/2 = (3+51)/2 = (5+49)/2 = 27

Sum of the sequence = the number of terms(27) multiplied by the average term value(27) = 27*27 = 729

729 3 \sqrt[3]{729} = 9

Cs ಠ_ಠ Lee
May 24, 2015

lod dis is ez 53 = 27 th odd number 27^2 = 1+ 3 + 5.... + 53

27 = 3^3 (3^3)^2 = 3^6

(3^6 )^1/3 = 3 ^2

9

Rama Devi
May 13, 2015

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