Find the smallest possible value of the middle number.

Let a, b, c, d and e be consecutive positive integers such that

b + c + d is a perfect square

and

a + b + c + d + e is a perfect cube.

Find the smallest possible value of c


The answer is 675.

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1 solution

Marco Brezzi
Aug 8, 2017

Since a , b , c , d , e a,b,c,d,e are consecutive positive integers

a = n 2 , b = n 1 , c = n , d = n + 1 , e = n + 2 n Z > 2 a=n-2,b=n-1,c=n,d=n+1,e=n+2 \quad n\in\mathbb{Z}_{>2}

We have two conditions

{ b + c + d = p 2 a + b + c + d + e = q 3 p , q Z \begin{cases} b+c+d=p^2\\ a+b+c+d+e=q^3 \end{cases} \qquad p,q\in\mathbb{Z}

Which translate to

{ 3 n = p 2 5 n = q 3 \begin{cases} 3n=p^2\\ 5n=q^3 \end{cases}

In the first equation 3 n 3n is divisible by 3 3 , so p p is divisible by 3 3 too, it follows that p 2 p^2 is divisible by 9, which implies that n n is divisible by 3 3 . With a similar reasoning on the second equation, n n is divisible by 25 25 . We can now write n = 75 k c ( k N ) n=75k \phantom{c} (k\in\mathbb{N}) , substitute in the original system

{ 225 k = p 2 125 3 k = q 3 \begin{cases} 225k=p^2\\ 125\cdot 3k=q^3 \end{cases}

From the first equation k k must be a square, from the second 3 k 3k must be a cube, therefore the smallest such k k is 9 9

To conclude, the answer is

c = n = 75 k = 75 9 = 675 c=n=75k=75\cdot 9 = \boxed{675}

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