Proving that vertical angles are equal.

How do you know vertical angles are congruent?

When two lines intersect to make an X, angles on opposite sides of the X are called vertical angles. These angles are equal, and here's the official theorem that tells you so. Vertical angles are congruent: If two angles are vertical angles, then they're congruent (see the above figure).

Do vertical angles make triangles congruent?

7. Altitude of a Triangle - a segment from any vertex of a Δ perpendicular to the line containing the opposite side. Vertical angles are congruent.