Browse · harp
Printsmc
number theory senior
Problem
How many positive two-digit integers are factors of ?
(A)
(B)
(C)
(D)
Solution
Repeating difference of squares: The sum of cubes formula gives us: A quick check shows is prime. Thus, the only factors to be concerned about are , since multiplying by will make any factor too large. Multiplying by or will give a two-digit factor; itself will also work. The next smallest factor, , gives a three-digit number. Thus, there are factors that are multiples of . Multiplying by , , or will also give a two-digit factor, as well as itself. Higher numbers will not work, giving additional factors. Multiply by , , or for a two-digit factor. There are no more factors to check, as all factors which include are already counted. Thus, there are an additional factors. Multiply by or for a two-digit factor. All higher factors have been counted already, so there are more factors. Thus, the total number of factors is
Final answer
D