Forgot the name of what these angles are called, help!

[FridayPanda]

I can't remember what angles α and β are called in relation to each other. It's like alternate angles but instead of two parallel lines it is only one. I've got a math test tomorrow and can't forget such a simple thing. Help!

 

[HallsofIvy]

Those are called "opposite angles" and there is a basic theorem in geometry that says "opposite angles are congruent".

 

[pka]

I have always know those as vertical angles.

 

[HallsofIvy]

That' probably more common, and so more accurate, than my answer.

 

[FridayPanda]

I have always know those as vertical angles.

thank you! exactly what i was looking for