Difference between revisions of "1999 USAMO Problems/Problem 3"
(Created page with "== Problem == Let <math>p > 2</math> be a prime and let <math>a,b,c,d</math> be integers not divisible by <math>p</math>, such that <cmath> \left\{ \dfrac{ra}{p} \right\} + \left...") |
Tigershark22 (talk | contribs) (→Solution) |
||
(4 intermediate revisions by 3 users not shown) | |||
Line 6: | Line 6: | ||
== Solution == | == Solution == | ||
− | {{ | + | We see that <math>\{\frac{ra+rb+rc+rd}{p}\}=0</math> means that <math>p|r(a+b+c+d)</math>. Now, since <math>p</math> does nto divide <math>r</math> and <math>p</math> is prime, their GCD is 1 so <math>p|a+b+c+d</math>. |
+ | |||
+ | Since <math>\{ \frac{ra}{p} \}+\{ \frac{rb}{p} \}+\{ \frac{rc}{p} \}+\{ \frac{rd}{p} \} =2</math>, then we see that they have to represent mods <math>\mod p</math>, and thus, our possible values of <math>p</math> are all such that <math>k^4 \equiv 1 \mod(p)</math> for all <math>k</math>. This happens when <math>p=3</math> or <math>5</math>. | ||
+ | |||
+ | When <math>p=3</math> then <math>r</math> is not divisible by 3, thus two are <math>1</math>, and the other two are <math>2</math>. Thus, four pairwise sums sum to 3. | ||
+ | |||
+ | When <math>p=5</math> then <math>r</math> is not divisible by 5 so <math>a, b, c, d</math> are <math>1, 2, 3</math> and <math>4</math>, so two pairwise sums sum to 5. | ||
+ | |||
+ | All three possible cases work so we are done. | ||
+ | |||
+ | (This solution makes absolutely no sense. Why does <math>k^4\equiv 1</math>??) | ||
== See Also == | == See Also == | ||
Line 12: | Line 22: | ||
[[Category:Olympiad Number Theory Problems]] | [[Category:Olympiad Number Theory Problems]] | ||
+ | {{MAA Notice}} |
Revision as of 11:50, 10 July 2020
Problem
Let be a prime and let be integers not divisible by , such that for any integer not divisible by . Prove that at least two of the numbers , , , , , are divisible by . (Note: denotes the fractional part of .)
Solution
We see that means that . Now, since does nto divide and is prime, their GCD is 1 so .
Since , then we see that they have to represent mods , and thus, our possible values of are all such that for all . This happens when or .
When then is not divisible by 3, thus two are , and the other two are . Thus, four pairwise sums sum to 3.
When then is not divisible by 5 so are and , so two pairwise sums sum to 5.
All three possible cases work so we are done.
(This solution makes absolutely no sense. Why does ??)
See Also
1999 USAMO (Problems • Resources) | ||
Preceded by Problem 2 |
Followed by Problem 4 | |
1 • 2 • 3 • 4 • 5 • 6 | ||
All USAMO Problems and Solutions |
The problems on this page are copyrighted by the Mathematical Association of America's American Mathematics Competitions.