## Vertical angles are congruent.

### Vertical angles are a pair of angles that share a common vertex and are formed by the intersection of two lines or line segments

Vertical angles are a pair of angles that share a common vertex and are formed by the intersection of two lines or line segments. In this context, congruent means that the two vertical angles have the same measure or size.

To understand why vertical angles are congruent, we need to look at the properties of intersecting lines or line segments. When two lines intersect, they form four angles at the point of intersection. Opposite angles, also known as vertical angles, are formed by one pair of opposite rays connecting the end points of the intersecting lines.

The key concept to note here is that opposite angles are formed by the same pair of opposite rays. This means that they have the same initial and terminal sides. Since the measure of an angle is determined by the amount of rotation from the initial side to the terminal side, it follows that vertical angles have equal measures.

In other words, if we have vertical angles ∠A and ∠B, and denote their measures as m∠A and m∠B respectively, then m∠A = m∠B. This equation holds true for any pair of vertical angles.

This property of vertical angles being congruent is useful in solving problems involving angles. For example, if we know that one vertical angle measures 50 degrees, we can immediately conclude that the opposite angle formed is also 50 degrees.

To summarize, when two lines or line segments intersect, the vertical angles formed are always congruent, meaning they have the same measure. This property arises from the fact that vertical angles are formed by the same pair of opposite rays.

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