2009 UNCO Math Contest II Problems/Problem 4

Problem

How many perfect squares are divisors of the product $1!\cdot 2!\cdot 3!\cdot 4!\cdot 5!\cdot 6!\cdot 7!\cdot 8!$ ? (Here, for example, $4!$ means $4\cdot 3\cdot 2\cdot 1$ )

Solution

We first factorize the product as $2^7\cdot3^2\cdot5\cdot7$ . Since we want only perfect squares, we are looking for even powers in the prime factorization of the divisors. Working with each term in the prime factorization, we find that there are four even powers of two that are less than or equal to $2^7$ , namely $2^0$ , $2^2$ , $2^4$ , $2^6$ , as $0$ is even. Repeating this process with three, five, and seven, we find that three has two even powers, $3^0$ , and $3^2$ , and that five and seven have only one even power, $5^0$ and $7^0$ respectively. Multiplying this we have $4\cdot2\cdot1\cdot1 = 8$ . Therefore, our answer is $8$ .

2009 UNCO Math Contest II (Problems • Answer Key • Resources)
Preceded by Problem 3	Followed by Problem 5
1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10
All UNCO Math Contest Problems and Solutions

Page

Toolbox

Search

2009 UNCO Math Contest II Problems/Problem 4

Problem

Solution

See also