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Good. Can you find the smallest odd integer n > 5 3 such that 3 1 + 3 + 5 + … + ( n − 2 ) + n is an integer?
In response to Challenge Master: From the solution we can conclude that n must be a perfect cube to obtain an integer from the expression. The next cube after 2 7 is 6 4 . a n = 1 + ( 6 4 − 1 ) 2 = 1 + 6 3 ( 2 ) = 1 2 7 .
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Yeah, you are right. :)
Meh. Nice one! :)
Good point rohit:-)
Note first that, in general for any integer n ≥ 1 ,
k = 1 ∑ n ( 2 k − 1 ) = 2 k = 1 ∑ n k − k = 1 ∑ n 1 = 2 ∗ 2 n ( n + 1 ) − n = n ( n + 1 ) − n = n 2 .
In this case, since 5 3 = 2 ∗ 2 7 − 1 , we see that the given expression is
( 2 7 2 ) 3 1 = ( 2 7 3 1 ) 2 = 3 2 = 9 .
Sum of n odd numbers is n 2 . 3 1 + 3 + 5 + ⋯ + 5 3 3 2 7 2 9
It would be better to explain why the sum of the first n odd numbers is equals to n 2 . Can you prove this statement? Hint: Arithmetic progression sum.
( 1 + 5 3 ) + ( 3 + 5 1 ) + ( 5 + 4 9 ) + … + ( 2 5 + 2 9 ) + ( 2 7 )
Can you take it from here?
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
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
Here is the simple common sense solution:
For the sequence, 1+3+5+...+53.
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
3 7 2 9 = 9
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
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3 1 + 3 + 5 + . . . . . + 5 3
The difference between two consecutive terms is same therefore it an A.P. with common difference 2 .
Let 5 3 be the n t h term of the sequence.
5 3 = 1 + ( n − 1 ) 2
n = 2 7
The sum of the sequence is 2 2 7 ( 1 + 5 3 ) or 2 7 2 .
We are required to find 3 2 7 2 which is,
9