1967 AMC 12 Problem 36
Attempt Problem 36 of the 1967 AMC 12 below, then check your answer against the professionally curated solution from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 1967 AMC 12 solutions, or check the answer key.
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36.
Given a geometric progression of five terms, each a positive integer less than The sum of the five terms is If is the sum of those terms in the progression which are squares of integers, then is:
Answer: C
Small Hint:
Write the rational common ratio in lowest terms as
Big Hint:
Integrality forces the middle term to be a multiple of ; use that is prime
Solution:
Let the common ratio be in lowest terms and the middle term be Since all five terms are integers, is divisible by so write The sum is then divisible by Thus is or But the middle term is less than so
Thus The bound gives If either is the possible sums are or not Coprimality therefore leaves and equal to and in either order. The progression is Its square terms sum to
Therefore, the correct answer is C.
Problem 36 in Other Years
1950 AMC 12 · 1951 AMC 12 · 1952 AMC 12 · 1953 AMC 12 · 1954 AMC 12 · 1955 AMC 12 · 1956 AMC 12 · 1957 AMC 12 · 1958 AMC 12 · 1959 AMC 12 · 1960 AMC 12 · 1961 AMC 12 · 1962 AMC 12 · 1963 AMC 12 · 1964 AMC 12 · 1965 AMC 12 · 1966 AMC 12