2003 AIME I Problems/Problem 8
In an increasing sequence of four positive integers, the first three terms form an arithmetic progression, the last three terms form a geometric progression, and the first and fourth terms differ by . Find the sum of the four terms.
Denote the first term as , and the common difference between the first three terms as . The four numbers thus are in the form .
Since the first and fourth terms differ by , we have that . Multiplying out by the denominator, This simplifies to , which upon rearranging yields .
Both and are positive integers, so and must have the same sign. Try if they are both positive (notice if they are both negative, then and , which is a contradiction). Then, . Directly substituting and testing shows that , but that if then . Alternatively, note that or implies that , so only may work. Hence, the four terms are , which indeed fits the given conditions. Their sum is .
As another option, could be rewritten as follows:
This gives another way to prove , and when rewritten one last time:
shows that must contain a factor of 3.
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