Square Divisors

6128704063125000 = 2 3 × 3 5 × 5 7 × 7 9 6128704063125000=2^3\times 3^5 \times 5^7 \times 7^9

For the number above, find the number of divisors which are perfect squares .


The answer is 120.

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2 solutions

Rushikesh Jogdand
Mar 22, 2016

Fact :- Degree of each prime factor in factorization of a perfect square number is even. \boxed{\text{Degree of each prime factor in factorization of a perfect square number is even.}}

Hence, we want to find all factors of given number in which degree of every prime factor is even.

Hence we count as follows-

total number of factors = choices for power of 2 × choices for power of 3 × choices for power of 5 × choices for power of 7 = 2 × 3 × 4 × 5 = 120 \begin{aligned} \text{total number of factors} & = & \text{choices for power of 2}\times \\ & & \text{choices for power of 3}\times \\ & & \text{choices for power of 5}\times \\ & & \text{choices for power of 7} \\ & = & 2\times 3\times 4\times 5 \\ & = & \boxed { 120 } \end{aligned}


*Two choices for powers of two are 2 0 or 2 2 2^{0}\text{ or } 2^{2}

short and sweet solution...(+!)

sakshi rathore - 5 years, 2 months ago

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Someone has said keep it simple

Rushikesh Jogdand - 5 years, 2 months ago

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someone of brilliant????..good..

sakshi rathore - 5 years, 2 months ago

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@Sakshi Rathore SRK -Happy New Year

Rushikesh Jogdand - 5 years, 2 months ago

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@Rushikesh Jogdand this is your new year!!!!!!....cool sm to you....In india today is a festival....

sakshi rathore - 5 years, 2 months ago

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@Sakshi Rathore Well, I meant that keep it simple is dialogue of ShahRukh Khan in movie Happy new year .
Happy New Year!!

Rushikesh Jogdand - 5 years, 2 months ago

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@Rushikesh Jogdand oh lol!!!!!!!!!!!! i was thinking you're from outside......but cool to know we are INDIANS.......btw 1 question do you make such good problems on your own or its from a book?

sakshi rathore - 5 years, 2 months ago

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@Sakshi Rathore Actually this is copied from a test paper. This was a new kind of problem than I've seen before. So, what place is better to share it than Brilliant !

Rushikesh Jogdand - 5 years, 2 months ago

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@Rushikesh Jogdand have you passed 12th.......or you are still preparing for any competitive exam....?

sakshi rathore - 5 years, 2 months ago

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@Sakshi Rathore Technically I've not passes 12th yet (Results are yet to come):) I'm preparing for JEE.

Rushikesh Jogdand - 5 years, 2 months ago

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@Rushikesh Jogdand Do you know about Brilliant Lounge?

Rushikesh Jogdand - 5 years, 2 months ago

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@Rushikesh Jogdand no what's it? do you know....

sakshi rathore - 5 years, 2 months ago

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@Sakshi Rathore It's messaging platform for Brilliant users.

Rushikesh Jogdand - 5 years, 2 months ago

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)(Rishikesh Jogdand is correct

Adolphout H - 1 year ago

You are wrong !!!!!!

Dustin Jourdan - 4 years, 2 months ago

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I agree with you :)

Dustin Jourdan - 4 years, 2 months ago

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Me either dude

Dustin Jourdan - 4 years, 2 months ago

Please elaborate.

Rushikesh Jogdand - 4 years, 2 months ago

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No he is correct

Adolphout H - 1 year ago
Vu Vincent
Jul 10, 2017

First, we list all possible 2 n 2n powers of a prime in this factorization ( because if 2 n 2n powers, then we have a certain prime factor p 2 n = ( p n ) 2 p^{2n} = (p^n)^2 suggesting a perfect square for a certain p n p^n );

1 2 2 2^2
1 3 2 3^2 3 4 3^4
1 5 2 5^2 5 4 5^4 5 6 5^6
1 7 2 7^2 7 4 7^4 7 6 7^6 7 8 7^8

We have a total of 2 × 3 × 4 × 5 = 120 2 \times 3 \times 4 \times 5\ = \boxed{120} divisors

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