Difference between revisions of "1987 AIME Problems/Problem 8"
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What is the largest positive integer <math>\displaystyle n</math> for which there is a unique integer <math>\displaystyle k</math> such that <math>\displaystyle \frac{8}{15} < \frac{n}{n + k} < \frac{7}{13}</math>? | What is the largest positive integer <math>\displaystyle n</math> for which there is a unique integer <math>\displaystyle k</math> such that <math>\displaystyle \frac{8}{15} < \frac{n}{n + k} < \frac{7}{13}</math>? | ||
== Solution == | == Solution == | ||
| − | {{ | + | Multiplying out all of the [[denominator]]s, we get: |
| + | |||
| + | :<math>104(n+k) < 195n < 105(n+k)</math> | ||
| + | :<math>0 < 91n - 104k < n + k</math> | ||
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| + | Since <math>91n - 104k < n + k</math>, <math>k > \frac{6}{7}n</math>. Also, <math>0 < 91n - 104k</math>, so <math>k < \frac{7n}{8}</math>. Thus, <math>48n < 56k < 49n</math>. <math>k</math> is unique if it is within a maximum [[range]] of 112, so <math>n = 112</math>. | ||
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== See also == | == See also == | ||
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{{AIME box|year=1987|num-b=7|num-a=9}} | {{AIME box|year=1987|num-b=7|num-a=9}} | ||
Revision as of 13:15, 11 February 2007
Problem
What is the largest positive integer 
for which there is a unique integer 
such that 
?
Solution
Multiplying out all of the denominators, we get:
Since 
, 
. Also, 
, so 
. Thus, 
. 
is unique if it is within a maximum range of 112, so 
.
See also
| 1987 AIME (Problems • Answer Key • Resources) | ||
| Preceded by Problem 7 |
Followed by Problem 9 | |
| 1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 | ||
| All AIME Problems and Solutions | ||
