Bunuel
The value of \((\sqrt{8!} + \sqrt{9!})^2\) is an integer. What is the greatest integer n such that \(2^n\) is a factor of \((\sqrt{8!} + \sqrt{9!})^2\)?
A. 3
B. 6
C. 8
D. 11
E. 14
(√8! + √9!)^2 = 8! + 2√(8!×9!) + 9! =
8! + 2√(8!×8!×9) + 9! =
8! + 2×8!×3 + 9! =
8!(1 + 6 + 9) =
8!×16 =
8!×2^4
We need to find the greatest integer n such that (8!×2^4)/2^n = integer. The expression above is an integer if n is not greater than the total number of 2s in the prime factored form of the numerator.
We can quickly determine the total number of 2s in the prime factored form of 8! with the following technique:
8/2 = 4; 4/2 = 2; 2/2 = 1
The total number of 2s is 4 + 2 + 1 = 7, so we have:
n ≤ 7 + 4 [We add 4 because there are 4 additional 2s in 2^4.]
n ≤ 11
We see that the maximum value of integer n is 11.
Answer: D