Find the smallest integer n > 1 such that the average of
1 , 2 , 3 , 4 , … , n
is a perfect square.
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Nice systematic solution !
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...
nice
1 + 2 + 3 + 4 . . . . . . + n = 2 n × ( n + 1 )
n 1 + 2 + 3 + 4 . . . . + n = 2 n + 1
We will get smallest integer when 2 n + 1 is equal to smallest perfect square greater than 1.
2 n + 1 = 4
n + 1 = 8
n = 7
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 , since 7 1 + 2 + 3 + . . . + 7 = 4 . Thus, n = 7 .
the method is correct but the answer is 7
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Indeed it is. Thanks for the correction.
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I have updated your solution. Remember to check for careless mistakes.
The average is just 2 n + 1 = p 2 ⟹ 2 p 2 − 1 = n . Now if p is 1, n=0, absurd. If p = 2, then we have n = 7 .
Lemma: 1 + 2 + 3 + . . . + n = 2 n ( n + 1 ) for all natural numbers n .
I will prove this by induction. Base case: n = 1 :
1 = 2 1 ( 1 + 1 ) . Inductive step: assume the equation holds for n = k for some integer k . Plugging in k + 1 for n :
1 + 2 + 3 + . . . + k + k + 1 = 2 ( k ) ( k + 1 ) + k + 1 by the assumption.
= 2 ( k ) ( k + 1 ) + 2 ( k + 1 )
= 2 ( k + 1 ) ( k + 2 ) . Thus the equation holds for n = k + 1 and so the lemma is complete.
The average of the n numbers is thus 2 n n ( n + 1 ) = 2 n + 1 . Seeing that n > 4 , the lowest integer value of n that produces a perfect square is 7 .
(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.
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
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
(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
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
n 1 + 2 + 3 + 4 + . . . + n = n ( n + 1 ) 2 n = 2 n + 1 = x 2
x is any positive integer.
To find the smallest integer n , we need to minimize the value of x 2 . n > 1 , then x > 1 .So the minimum value of x 2 = 2 2 = 4 .
We have the value of x 2 , let's solve the equation.
4 = 2 n + 1
n + 1 = 4 × 2 = 8
n = 8 − 1 = 7 .The smallest integer n is 7 .
Average of first n integers is sum of them divided by n . Which is...
n 1 ∑ n = n 1 2 n ⋅ ( n + 1 ) = 2 n + 1 = k 2
Since, n > 1 , k = 1 . So we try the next smallest k which is 2 . Now we get...
2 n + 1 = 2 2 ∴ n = 7
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
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 :)
1+2+3+4........+7=28/7=4 which is square of 2.
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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Let the Perfect Square be x 2
Now, since the sum of n numbers are 2 n × ( n + 1 )
The average of n will be 2 ( n + 1 )
So we get,
2 ( n + 1 ) = x 2
or, n = 2 x 2 − 1
For x = 1 the value of n will be 1
Since, n > 1
The next smallest value of n can be found if x = 2
So, for x = 2 we get
n = 2 × 2 2 − 1 = 7
Thus, n = 7