Square peg

Find the smallest integer n > 1 n>1 such that the average of

1 , 2 , 3 , 4 , , n 1, 2, 3, 4, \ldots , n

is a perfect square.


The answer is 7.

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16 solutions

Tahsin Faruque
Dec 8, 2013

Let the Perfect Square be x 2 x^{2}

Now, since the sum of n n numbers are n × ( n + 1 ) 2 \frac{n \times (n+1)}{2}

The average of n n will be ( n + 1 ) 2 \frac{(n+1)}{2}

So we get,

( n + 1 ) 2 = x 2 \frac{(n+1)}{2} = x^{2}

or, n = 2 x 2 1 n = 2 x^{2} - 1

For x = 1 x=1 the value of n n will be 1 1

Since, n > 1 n>1

The next smallest value of n n can be found if x = 2 x=2

So, for x = 2 x=2 we get

n = 2 × 2 2 1 = 7 n= 2 \times 2^{2} - 1 = 7

Thus, n = 7 \boxed{n=7}

Nice systematic solution !

Upendra Singh - 7 years, 6 months ago

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Thanks!

Tahsin Faruque - 7 years, 6 months ago

1+2+3+4+5+6+7 ? <--- what i thought it meant = 28 which equals 5.29 where as 1+2+3+4+5+6+7+8 = 36 which is perfect square so i came up with 8 on the number line n = 8>1 ehhhh...

Dewayne Gunter - 7 years, 6 months ago

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Ah, sorry for the bad luck :(

Tahsin Faruque - 7 years, 6 months ago

nice

Abhishek Aggarwal - 7 years, 6 months ago

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Thanks! :)

Tahsin Faruque - 7 years, 6 months ago
Anish Shah
Dec 13, 2013

1 + 2 + 3 + 4...... + n = n × ( n + 1 ) 2 1+2+3+4......+n = \frac{n \times (n+1)}{2}

1 + 2 + 3 + 4.... + n n = n + 1 2 \frac{1+2+3+4....+n}{n} = \frac{n+1}{2}

We will get smallest integer when n + 1 2 \frac{n+1}{2} is equal to smallest perfect square greater than 1.

n + 1 2 = 4 \frac{n+1}{2} = 4

n + 1 = 8 n+1 = 8

n = 7 n = \boxed{7}

Gustavo Chicato
Dec 8, 2013

A perfect square is, by definition, an integer that is the square of an integer. The first such number greater than 1 is 4, which can be written as an average of 1 , 2 , 3 , . . . , 7 {1, 2, 3, ..., 7} , since 1 + 2 + 3 + . . . + 7 7 = 4 \frac{1+2+3+...+7}{7} = 4 . Thus, n = 7 \boxed{7} .

the method is correct but the answer is 7

GzeXie Cheong - 7 years, 6 months ago

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Indeed it is. Thanks for the correction.

Gustavo Chicato - 7 years, 6 months ago

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I have updated your solution. Remember to check for careless mistakes.

Calvin Lin Staff - 7 years, 6 months ago
Sameer L.
Dec 23, 2013

The average is just n + 1 2 = p 2 2 p 2 1 = n \dfrac{n+1}{2} = p^2 \implies 2p^2 - 1 = n . Now if p is 1, n=0, absurd. If p = 2, then we have n = 7 n =\boxed{7} .

Zachary Chase
Dec 9, 2013

Lemma: 1 + 2 + 3 + . . . + n = n ( n + 1 ) 2 1+2+3+ ... + n = \frac{n(n+1)}{2} for all natural numbers n n .

I will prove this by induction. Base case: n = 1 n=1 :

1 = 1 ( 1 + 1 ) 2 1 = \frac{1(1+1)}{2} . Inductive step: assume the equation holds for n = k n=k for some integer k k . Plugging in k + 1 k+1 for n n :

1 + 2 + 3 + . . . + k + k + 1 = ( k ) ( k + 1 ) 2 + k + 1 1+2+3+...+k+k+1 = \frac{(k)(k+1)}{2} + k+1 by the assumption.

= ( k ) ( k + 1 ) + 2 ( k + 1 ) 2 = \frac{(k)(k+1)+2(k+1)}{2}

= ( k + 1 ) ( k + 2 ) 2 = \frac{(k+1)(k+2)}{2} . Thus the equation holds for n = k + 1 n = k+1 and so the lemma is complete.

The average of the n n numbers is thus n ( n + 1 ) 2 n = n + 1 2 \frac{n(n+1)}{2n} = \frac{n+1}{2} . Seeing that n > 4 n > 4 , the lowest integer value of n n that produces a perfect square is 7 \boxed{7} .

Eunice Stars
Dec 9, 2013

(1 + 2 +...+ n) : n = a^2 ---> ( n(n+1)/2) : n=a^2 ---> n(n+1)/2n = a^2 --->(n+1)/2 = a^2 --->n+1 = 2 . a^2 If a=1, n=1( unsuitable because n>1). If a=2, n=7 ( suitable ). My answer is 7.

Sanatan Samkalp
Dec 15, 2013

SMALLEST N = AVERAGE OF THAT SERIES WHOSE AVERAGE IS SMALLEST PERFECT SQUARE (4) . SO THERE ARE THREE DIGITS BEFORE 4 THEN THERE MUST BE THREE DIGITS AFTER 4 SO ANSWER IS 7

Abhilash Reddy
Dec 11, 2013

sum of first n natural numbers is n(n+1)/2 so average of these is n(n+1)/2n which is (n+1)/2 so to be a perfect square under root of (n+1)/2 should be a integer so by trial and error 7 is the right answer

Siva Prasad
Mar 23, 2014

(n (n+1))/2n is the Average. This is = ( n+1)/2 The smallest square that bigger than 1 is "4". So (n+1)/2= 4 n+1= 4 2= 8 n=8- 1=7

The answer is 7

Dinesh Kuncham
Dec 14, 2013

1+2+3+.......+n=n(n+1)/2, average of 1,2,3...n=(1+2+3+.......+n)/n=(n+1)/2 ,first perfect square is 4 so n+1/2=4,n=7

We can write the equation

1 + 2 + 3 + 4 + . . . + n n = ( n + 1 ) n 2 n = n + 1 2 = x 2 \frac{1+2+3+4+...+n}{n}=\frac{(n+1)\frac{n}{2}}{n}=\frac{n+1}{2}=x^2

x x is any positive integer.

To find the smallest integer n n , we need to minimize the value of x 2 x^2 . n > 1 n>1 , then x > 1 x>1 .So the minimum value of x 2 = 2 2 = 4 x^2=2^2=4 .

We have the value of x 2 x^2 , let's solve the equation.

4 = n + 1 2 4=\frac{n+1}{2}

n + 1 = 4 × 2 = 8 n+1=4\times2=8

n = 8 1 = 7 n=8-1=\boxed{7} .The smallest integer n n is 7 \boxed{7} .

Jubayer Nirjhor
Dec 14, 2013

Average of first n n integers is sum of them divided by n n . Which is...

1 n n = 1 n n ( n + 1 ) 2 = n + 1 2 = k 2 \dfrac{1}{n}\sum n = \dfrac{1}{n}\dfrac{n\cdot(n+1)}{2}=\dfrac{n+1}{2}=k^2

Since, n > 1 n>1 , k 1 k\neq 1 . So we try the next smallest k k which is 2 2 . Now we get...

n + 1 2 = 2 2 n = 7 \dfrac{n+1}{2}=2^2 ~~~ \therefore n=\fbox{7}

Rik Ghosh
Dec 13, 2013

we see that sum of natural numbers upto n is equal to n(n+1)/2 and average is equal to (n(n+1)/2)/n i.e (n+1)/2 .

now (n+1)/2 = k^2 for some k and if we take k=2 then n is smallest which is 7 as n>1

Nandan Gowda
Dec 13, 2013

The average of the no.s 1,2,3,4,...n is n(n+1)/2 /n = n+1/2

by this equation as told in question we have to get the perfect square value for average of those numbers,

Now, in (n+1)/2 Change the value of n>1 so that we get a perfect square number.

We get ,

7+1/2 = 4 which is 2^2

Therefore n=7 :)

Abdul Basit
Dec 13, 2013

1+2+3+4........+7=28/7=4 which is square of 2.

Nikko Aggarao
Dec 12, 2013

2+3+4+n=? then think a number ,when you add it in that equation the sum is a smallest perfect square when n>1

Answer:7

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