Maths Olympiad Prep

Library / /91 of 299

Geometry Difficulty 6.2 National Olympiad Prove it Iran

By angle, we mean its vertex together with its two rays. Find the maximum value of nn such that we can put nn 6060^\circ angles on the plane, in a way that each pair of which has 4 intersection points.

Solution

Assume that two 6060^\circ angles with vertices AA and BB have four intersection points MM, NN, PP and QQ as shown in the figure.
Figure 1
We have
AKB^=KAP^+APB^+PBK^=30+APB^+30>60 \widehat{AKB} = \widehat{KAP} + \widehat{APB} + \widehat{PBK} = 30^\circ + \widehat{APB} + 30^\circ > 60^\circ
And
AKB^=180KAB^KBA^<1803030=120 \widehat{AKB} = 180^\circ - \widehat{KAB} - \widehat{KBA} < 180^\circ - 30^\circ - 30^\circ = 120^\circ

So the value of the angle between two bisectors is between 6060^\circ and 120120^\circ, meaning that the acute angle between two lines is greater than 6060^\circ. If we have three 6060^\circ angles with the above properties, then we should have three lines such that the value of the acute angle between each pair is greater than 6060^\circ, which is impossible, due to a triangle with three angles greater than 6060^\circ, or in the case of concurrency six angles greater than 6060^\circ on a point. The example for only two 6060^\circ angles is shown in the figure! ■

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: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.