Dear student
2^8 + 2^11 + 2^n
= 2^8 ( 1 + 2^3 + 2^(n-8) )
= 2^8 (9 + 2^(n-8) )
Now for this to be a perfect square,
9 + 2^(n-8) sbould be a perfect square.
Hence 9 + 16 = 25 is a perfect square.
2^(n-8) = 16 = 2^4
n-8 = 4
n = 12
Regards
Arun (askIITians forum expert)