What are angles formed by two intersecting lines and which are opposite of each other called?

Answer 1

All angles are formed by two intersecting lines. The pairs of angles opposite each other are called vertical angles. If two angles are vertical, they measure the exact same. Say you name the angles formed as A, B, C, and D. A and C are vertical and B and D are vertical. The angles next to each other formed by intersecting lines are supplementary, and add up to 180 degrees. That means A + B = 180 (since they are next to each other) and B+C = 180. Subtracting the first equation by the second equation gives us A + B - B - C = 180 - 180, which simplifies to A - C = 0, which further simplifies to A = C. The same can be said about B and D being equal.