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
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Since 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
We have two conditions
{ b + c + d = p 2 a + b + c + d + e = q 3 p , q ∈ Z
Which translate to
{ 3 n = p 2 5 n = q 3
In the first equation 3 n is divisible by 3 , so p is divisible by 3 too, it follows that p 2 is divisible by 9, which implies that n is divisible by 3 . With a similar reasoning on the second equation, n is divisible by 2 5 . We can now write n = 7 5 k c ( k ∈ N ) , substitute in the original system
{ 2 2 5 k = p 2 1 2 5 ⋅ 3 k = q 3
From the first equation k must be a square, from the second 3 k must be a cube, therefore the smallest such k is 9
To conclude, the answer is
c = n = 7 5 k = 7 5 ⋅ 9 = 6 7 5