Powers and Factors

Level 2

How many factors of 7 9999 7^{9999} are greater than 1000000?


The answer is 9992.

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1 solution

Stephen Mellor
Oct 24, 2017

We can rewrite 7 9999 = 7 × 7 × . . . × 7 × 7 7^{9999} = 7 \times 7 \times ... \times 7 \times 7 (9999 times).

As all of the prime factors are 7, the factors are 1 , 7 , 7 2 , 7 3 , . . . 7 9998 , 7 9999 1, 7, 7^2, 7^3, ... 7^{9998}, 7^{9999} .

We can obviously exclude 1 1 , leaving us with 9999 9999 factors.

The number of factors that are greater than the number = 9999 = 9999 - The number of factors that are less than or equal to the number.

We find that 7 7 < 1000000 7^7 < 1000000 and 7 8 > 1000000 7^8 > 1000000

Therefore: The number of factors that are greater than the number = 9999 7 = 9992 = 9999 - 7 = \boxed{9992}

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