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Geometry Difficulty 7.0 National olympiad, round 2 Prove it Argentina

Initially we have a paper triangle ABCABC such that BAC=120\angle BAC = 120^\circ. In the first step, we draw the angle bisectors of the three angles of the triangle, which intersect in II, and then using a pair of scissors we cut along segments AIAI, BIBI and CICI, obtaining 3 triangles: ABIABI, BCIBCI and CAICAI. In the second step we repeat the same procedure with the three triangles, that is: each one of those is cut into three smaller triangles cutting along the angle bisectors. At the end of the second step we have 9 triangles in total. This procedure continues in the same way until we complete 10 steps.
How many of the triangles at the end of the process have an angle of 120120^\circ?

Solution

Let CAB=2α\angle CAB = 2\alpha, ABC=2β\angle ABC = 2\beta and BCA=2γ\angle BCA = 2\gamma. Notice that α+β+γ=90\alpha+\beta+\gamma = 90^\circ, and if II is the incenter of ABC\triangle ABC we can compute AIB\angle AIB, BIC\angle BIC, CIA\angle CIA in terms of these variables.
Figure 1

Fact 1: The number of 6060^\circ angles in any step of the process is equal to the number of 120120^\circ angles in the next step.
Proof: It suffices to prove that each 6060^\circ angle generates a 120120^\circ angle, and that every 120120^\circ angle is generated by a 6060^\circ angle.
The first statement is clear: if, say, 2α=602\alpha = 60^\circ, then 90+α=12090^\circ + \alpha = 120^\circ. For the second statement, observe that the only way we can obtain a 120120^\circ angle is if one of the angles at I measures 120120^\circ, because those are the only obtuse angles generated. For this to be true, one of our variables must be equal to 3030^\circ, which in turn means one of the angles of ABC must be 6060^\circ.
Fact 2: The number of 120120^\circ angles in any step of the process is half the number of 6060^\circ angles in the next step.
Proof: When performing a step, every 120120^\circ angle gets divided into two 6060^\circ angles. Moreover, this is the only way of obtaining a 6060^\circ angle. \square
Using these two facts we can readily complete the following table, which shows the answer is 3232.

Step12345678910
Measure60°120°60°120°60°120°60°120°60°120°
Angles22448816163232

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