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 100.100. The sum of the five terms is 211.211. If SS is the sum of those terms in the progression which are squares of integers, then SS is:

00

9191

133133

195195

211211

Answer: C
Concepts:geometric sequencedivisibilityperfect squarecasework
Difficulty rating: 2380
Small Hint:

Write the rational common ratio in lowest terms as cd\frac{c}{d}

Big Hint:

Integrality forces the middle term to be a multiple of c2d2c^2d^2; use that 211211 is prime

Solution:

Let the common ratio be cd\frac{c}{d} in lowest terms and the middle term be a.a. Since all five terms are integers, aa is divisible by c2d2,c^2d^2, so write a=kc2d2.a=kc^2d^2. The sum 211211 is then divisible by k.k. Thus kk is 11 or 211.211. But the middle term aa is less than 100,100, so k=1.k=1.

Thus d4+d3c+d2c2+dc3+c4=211. \begin{aligned} d^4+d^3c+d^2c^2&\\ \quad+dc^3+c^4&=211. \end{aligned} The bound gives c,d<4.c,d\lt4. If either is 1,1, the possible sums are 5,5, 31,31, or 121,121, not 211.211. Coprimality therefore leaves cc and dd equal to 22 and 33 in either order. The progression is 16,16, 24,24, 36,36, 54,54, 81.81. Its square terms sum to 16+36+81=133.16+36+81=133.

Therefore, the correct answer is C.

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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