# Difference between revisions of "1991 AHSME Problems/Problem 12"

## Problem

The measures (in degrees) of the interior angles of a convex hexagon form an arithmetic sequence of integers. Let $m$ be the measure of the largest interior angle of the hexagon. The largest possible value of $m$, in degrees, is

(A) 165 (B) 167 (C) 170 (D) 175 (E) 179

## Solution

$\fbox{D}$ The angles must add to $180(6-2) = 720$. Now, let the arithmetic progression have first term $a$ and difference $d$, so by the sum formula, we get $\frac{6}{2}(2a+5d) = 720 \implies 2a+5d=240.$ Now, $240$ and $5d$ are both divisible by $5$ (as $d$ is an integer), so $2a$ is divisible by $5$, and thus $a$ is divisible by 5, so $m = a+5d$ is divisible by $5$. As the hexagon is convex, $m$ must be less than $180$, so as it is a multiple of $5$, it can be at most $175$, and indeed this is possible with $a = 65, d = 22.$