Difference between revisions of "2009 AIME I Problems/Problem 1"
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Revision as of 20:20, 4 July 2013
Problem
Call a -digit number geometric if it has
distinct digits which, when read from left to right, form a geometric sequence. Find the difference between the largest and smallest geometric numbers.
Solution
Solution 1
Assume that the largest geometric number starts with a nine. We know that the common ratio must be a rational of the form for some integer
, because a whole number should be attained for the 3rd term as well. When
, the number is
. When
, the number is
. When
, we get
, but the integers must be distinct. By the same logic, the smallest geometric number is
. The largest geometric number is
and the smallest is
. Thus the difference is
.
Solution 2
Consider the three-digit number . If its digits form a geometric sequence, we must have that
, that is,
.
The minimum and maximum geometric numbers occur when is minized and maximized, respectively. The minimum occurs when
; letting
and
achieves this, so the smallest possible geometric number is 124.
For the maximum, we have that ;
is maximized when
is the greatest possible perfect square; this happens when
, yielding
. Thus, the largest possible geometric number is 964.
Our answer is thus .
See also
2009 AIME I (Problems • Answer Key • Resources) | ||
Preceded by First Question |
Followed by Problem 2 | |
1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 | ||
All AIME Problems and Solutions |
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