# How do alternate angles add up to 180?

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- charlatanLv 71 month ago
alternate angles add up to 180

when two lines bisect each other at right angle.

- husoskiLv 71 month ago
The usage of "alternate angle" that I'm aware of is to mean either "alternate interior angles" or "alternate exterior angles".

If they are produced by a traversal of two parallel lines, then any two alternate interior angles are congruent and any two alternate exterior angles are congruent.

The only way for congruent angles to be supplementary ("add up to 180") is if they are both right angles. For that to be true, you need the traversal line to be perpendicular to the two parallel lines.

- JJLv 71 month ago
Well let's say each is 90, then 90+90=180. When one is 120 then the other is 60, and so on. But they always add up to 180.

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