Huge numbers

Which of the following divides 36 ! 36! ?

Notation :
! ! denotes the factorial notation. For example, 10 ! = 1 × 2 × 3 × × 10 10! = 1\times2\times3\times\cdots\times10 .

None of these choices 2 36 2^{36} 1 7 12 17^{12} 1 2 17 12^{17} 2 38 2^{38} 2 4 36 24^{36}

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1 solution

Akshat Sharda
Mar 10, 2016

Firstly, finding the highest power of 2 2 which divides 36 ! 36! ,

= 36 2 + 36 2 2 + 36 2 3 + 36 2 4 + 36 2 5 = 18 + 9 + 4 + 2 + 1 = 34 =\left \lfloor \frac{36}{2} \right \rfloor+\left \lfloor \frac{36}{2^2} \right \rfloor+\left \lfloor \frac{36}{2^3} \right \rfloor+\left \lfloor \frac{36}{2^4} \right \rfloor+\left \lfloor \frac{36}{2^5} \right \rfloor \\ = 18+9+4+2+1=34

So, 4 17 4^{17} also divides 36 ! 36! (this result will be helpful for us later in the solution).

Thus, option having 2 38 , 2 36 2^{38},2^{36} and 2 4 36 24^{36} are rejected.

Now, we can see that highest power 17 17 is 2 2 which can divide 36 ! 36! .

Now, let's find the highest power of 3 3 which can divide 36 ! 36! ,

= 36 3 + 36 3 2 + 36 3 3 = 12 + 4 + 1 = 17 =\left \lfloor \frac{36}{3} \right \rfloor+\left \lfloor \frac{36}{3^2} \right \rfloor+\left \lfloor \frac{36}{3^3} \right \rfloor \\ =12+4+1=17

As 4 17 4^{17} and 3 17 3^{17} both divide 36 ! 36! , therefore, 1 2 17 12^{17} also divides 36 ! 36! .

Almost the same way.

Niranjan Khanderia - 3 years, 7 months ago

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