Difference between revisions of "2005 Cyprus Seniors TST/Day 1/Problem 4"
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== Solution == | == Solution == | ||
+ | We first prove that there is a multiple of 2 and a multiple of 3 in three consecutive integers: | ||
+ | n, n+1, n+2 | ||
+ | |||
+ | If n is odd, n+1 is even, and therefore a multiple of 2. If n is even, n is a multiple of 2. | ||
+ | |||
+ | |||
+ | n can be a multiple of 3. If <math>n\equiv 1 \pmod 3</math>, then n+2 is a multiple of 3. If <math>n\equiv 2 \pmod 3</math>, n+1 is a multiple of 3. | ||
+ | |||
+ | Therefore, there is a multiple of 2 and a multiple of 3 in any three consecutive integers. Therefore, the product of three consecutive integers is a multiple of 6. | ||
+ | |||
+ | Now we prove the second: | ||
+ | |||
+ | <math>\nu=2x-1</math> | ||
+ | |||
+ | <math>(10x-4)(10x-2)10x</math> | ||
+ | |||
+ | 10 is in there. The argument used on the last theorem can be used here to get that 3 is in here. Now we just need to find an 8 in here. | ||
+ | |||
+ | We can use <math>(10x-4)(10x-2)</math>. We can divide out a 4: | ||
+ | |||
+ | <math>4(5x-2)(5x-1)</math> | ||
+ | |||
+ | We prove that there is a 2 in there, so there is an 8 in there. | ||
+ | |||
+ | 240 divides the product <math>(5\nu+1)(5\nu+3)(5\nu+5)</math>, where <math>\nu</math> is any odd number. | ||
Latest revision as of 20:12, 9 October 2007
Problem
Prove that number 6 divides the product of every three consecutive integer numbers and then show that 240 divides the product , where is any odd number.
Solution
We first prove that there is a multiple of 2 and a multiple of 3 in three consecutive integers:
n, n+1, n+2
If n is odd, n+1 is even, and therefore a multiple of 2. If n is even, n is a multiple of 2.
n can be a multiple of 3. If , then n+2 is a multiple of 3. If , n+1 is a multiple of 3.
Therefore, there is a multiple of 2 and a multiple of 3 in any three consecutive integers. Therefore, the product of three consecutive integers is a multiple of 6.
Now we prove the second:
10 is in there. The argument used on the last theorem can be used here to get that 3 is in here. Now we just need to find an 8 in here.
We can use . We can divide out a 4:
We prove that there is a 2 in there, so there is an 8 in there.
240 divides the product , where is any odd number.