Let S = 125! + 13! 23! + 15! 21! + up to 13 terms. If 13 S = 2kn!, k N, then n + k is equal to
- 50
- 52
- 49
- 51
Answer
(3) 49
Solution
The general term is 1r! (26 - r)! with r = 1, 3, 5, , 25 (13 terms), so
S = 126! [26C1 + 26C3 + + 26C25 ]
The sum of the odd binomial coefficients is 226 - 1 = 225, so
S = 22526!
13 S = 13 22526 25! = 22425!
Hence k = 24 and n = 25.
n + k = 49
Answer: (3)