Maths Olympiad Prep

Library / /336 of 520

Geometry Difficulty 3.3 AMC 10/12 Find the answer

Mary divides a circle into 12 sectors. The central angles of these sectors, measured in degrees, are all integers and they form an arithmetic sequence. What is the degree measure of the smallest possible sector angle?

Pick one

Solution

Let a1a_1 be the first term of the arithmetic progression and a12a_{12} be the last term of the arithmetic progression. From the formula of the sum of an arithmetic progression (or arithmetic series), we have 12a1+a122=36012*\frac{a_1+a_{12}}{2}=360, which leads us to a1+a12=60a_1 + a_{12} = 60. a12a_{12}, the largest term of the progression, can also be expressed as a1+11da_1+11d, where dd is the common difference. Since each angle measure must be an integer, dd must also be an integer. We can isolate dd by subtracting a1a_1 from a12a_{12} like so: a12a1=a1+11da1=11da_{12}-a_1=a_1+11d-a_1=11d. Since dd is an integer, the difference between the first and last terms, 11d11d, must be divisible by 11.11. Since the total difference must be less than 6060, we can start checking multiples of 1111 less than 6060 for the total difference between a1a_1 and a12a_{12}. We start with the largest multiple, because the maximum difference will result in the minimum value of the first term. If the difference is 5555, a1=60552=2.5a_1=\frac{60-55}{2}=2.5, which is not an integer, nor is it one of the five options given. If the difference is 4444, a1=60442a_1=\frac{60-44}{2}, or (C) 8\boxed{\textbf{(C)}\ 8}
-Solution by Rhiju

Want a route through all this instead of an archive? The track puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.

Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.