2013 AMC 12B Problem 13

Attempt Problem 13 of the 2013 AMC 12B 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 2013 AMC 12B solutions, or check the answer key.

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13.

The internal angles of quadrilateral ABCDABCD form an arithmetic progression. Triangles ABDABD and DCBDCB are similar with DBA=DCB\angle DBA = \angle DCB and ADB=CBD.\angle ADB = \angle CBD. Moreover, the angles in each of these two triangles also form an arithmetic progression. In degrees, what is the largest possible sum of the two largest angles of ABCD?ABCD?

210210

220220

230230

240240

250250

Answer: D
Concepts:arithmetic sequenceangle chasingcasework
Difficulty rating: 1700
Small Hint:

A triangle’s angles form an arithmetic progression exactly when its middle angle is 6060^\circ

Big Hint:

Let DBA=x\angle DBA = x and ADB=y;\angle ADB = y; the four angles of ABCDABCD become x,y,180y,180x,x, y, 180-y, 180-x, which must be an arithmetic progression with one triangle angle equal to 6060^\circ

Solution:

The angles of a triangle form an arithmetic progression exactly when the middle one is 60.60^\circ. With DBA=x\angle DBA = x and ADB=y,\angle ADB = y, the four angles of ABCDABCD are x,y,180y,180x,x, y, 180 - y, 180-x, which must itself be an arithmetic progression. In increasing order they are either x,y,180y,180xx,y,180-y,180-x or x,180y,y,180x,x,180-y,y,180-x, giving 3y=x+1803y=x+180 or 3y=360x.3y=360-x. One of the triangle angles x,y,180xyx,y,180-x-y is 60.60^\circ. Substitution leaves the angle sets (60,80,100,120)(60,80,100,120) and (45,75,105,135).(45,75,105,135). The two largest angles sum to at most 105+135=240.105 + 135 = 240. Thus, the correct answer is D.

Problem 12#12
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