Question

Prove that the following statements are false provide counter examples to disprove them: (1) If two angles have common vertex and their bisectors create one line, then these angles are vertical. (2) If the angle bisectors of two equal angles belong to the same line, then these angles are vertical.

Answer 1

Examples to disprove them are as follows;

1) Angles of two different inscribed regular polygon in contact at their vertices

2) Angles on the opposite corners of a regular even sided polygon

Explanation:

The statement to be disproved can be presented as follows;

1) ∠A and ∠B share a common vertex

2) The bisectors of ∠A and ∠B create one line then

∠A and ∠B are vertical, that is ∠A and ∠B are on the opposite sides of the point where two lines cross or ∠A = ∠B

Given that ∠A = Interior angle of a inscribed square = 90°

∠B = Interior angle of an inscribed equilateral triangle = 60°

Therefore the two circles can be arranged such that the line connecting the centers of both circles, are the bisectors of ∠A and ∠B

However, ∠A ≠ ∠B hence the two angles are not vertical QED

(2) The angles ∠A and ∠B where ∠A = ∠B are on the opposite corners of an even sided regular polygon are equal and their bisectors belong to the same line, however, it can be shown that ∠A and ∠B are not vertical.