There are exactly four positive integers $n$ such that \[\frac{(n + 1)^2}{n + 23}\] is an integer. Compute the largest such $n$.

Respuesta :

The given expression is
(n+1)²/(n+23)
This s equivalent to
(n² + 2n + 1)/(n+23)

Perform long division.
                      n - 21
         -----------------------
n+23 | n² +  2n + 1
           n² + 23n
          -----------------------
                  -21n + 1
                  -21n - 483
                 ----------------
                            484
The given expression is equivalent to 
n - 21 + 484/(n+23)

To obtain an integer, we want 484/(n+23) to be a factor of 484.
Factors of 484 are
484 = 2*242
        = 4*121
        = 4*11*11 = 11*44
 
Therefore, obtain
n+23 = 484  => n = 461
n+23 = 242 => n = 219
 and so on, with decreasing values of n.

Answer:
The largest value of n is 461.