1961 AMC 12 Problem 32

Attempt Problem 32 of the 1961 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 1961 AMC 12 solutions, or check the answer key.

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32.

A regular polygon of nn sides is inscribed in a circle of radius R.R. The area of the polygon is 3R2.3R^2. Then nn equals:

88

1010

1212

1515

1818

Answer: C
Concepts:regular polygontrigonometryarea decomposition
Difficulty rating: 1300
Small Hint:

Divide the polygon into nn congruent triangles with vertex at the center

Big Hint:

Use area 12nR2sin(360n)\frac12nR^2\sin(\frac{360^\circ}{n}) and test the choices

Solution:

The polygon’s area is n2R2sin360n. \frac n2R^2\sin\frac{360^\circ}{n}. For n=12,n=12, this becomes 6R2sin30=3R2, 6R^2\sin30^\circ=3R^2, as required.

Therefore, the correct answer is C.

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Problem 32 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 · 1962 AMC 12 · 1963 AMC 12 · 1964 AMC 12 · 1965 AMC 12 · 1966 AMC 12 · 1967 AMC 12 · 1968 AMC 12 · 1969 AMC 12 · 1970 AMC 12 · 1971 AMC 12 · 1972 AMC 12 · 1973 AMC 12