Difference between revisions of "2000 AIME I Problems/Problem 6"
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== Problem == | == Problem == | ||
− | For how many [[ordered pair]]s <math>(x,y)</math> of [[integer]]s is it true that <math>0 < x < y < 10^ | + | For how many [[ordered pair]]s <math>(x,y)</math> of [[integer]]s is it true that <math>0 < x < y < 10^3 |
+ | and that the [[arithmetic mean]] of </math>x<math> and </math>y<math> is exactly </math>2<math> more than the [[geometric mean]] of </math>x<math> and </math>y$? | ||
== Solution == | == Solution == |
Revision as of 04:25, 18 November 2017
Problem
For how many ordered pairs of integers is it true that
x
y
2
x
y$?
Solution
Solution 1
Because , we only consider
.
For simplicity, we can count how many valid pairs of that satisfy our equation.
The maximum that can be is
because
must be an integer (this is because
, an integer). Then
, and we continue this downward until
, in which case
. The number of pairs of
, and so
is then
.
Solution 2
Let =
and
=
Then
This makes counting a lot easier since now we just have to find all pairs that differ by 2.
Because , then we can use all positive integers less than 1000 for
and
.
Without loss of generality, let's say .
We can count even and odd pairs separately to make things easier*:
Odd:
Even:
This makes 499 odd pairs and 498 even pairs, for a total of pairs.
Note: We are counting the pairs for the values of
and
, which, when squared, translate to the pairs of
we are trying to find.
See also
2000 AIME I (Problems • Answer Key • Resources) | ||
Preceded by Problem 5 |
Followed by Problem 7 | |
1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 | ||
All AIME Problems and Solutions |
The problems on this page are copyrighted by the Mathematical Association of America's American Mathematics Competitions.