By angle, we mean its vertex together with its two rays. Find the maximum value of such that we can put angles on the plane, in a way that each pair of which has 4 intersection points.
Solution
Assume that two angles with vertices and have four intersection points , , and as shown in the figure.
We have
And
So the value of the angle between two bisectors is between and , meaning that the acute angle between two lines is greater than . If we have three 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 , which is impossible, due to a triangle with three angles greater than , or in the case of concurrency six angles greater than on a point. The example for only two angles is shown in the figure! ■
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