# Chapter 1 - Tools of Geometry - 1-5 Exploring Angle Pairs - Practice and Problem-Solving Exercises: 41

Vertical angles are supplementary when the lines intersect at right angles. Such lines are perpendicular.

#### Work Step by Step

The measures of supplementary angles sum to $180^o$. Vertical angles each have the same measure, so supplementary vertical angles must each measure $90^o$. $90^o$ angles are right angles, so the lines that intersect to form supplementary vertical angles must intersect at right angles.

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