standard_description: Prove theorems about lines and angles. Theorems include: vertical angles are congruent; when a transversal crosses parallel lines, alternate interior angles are congruent and corresponding angles are congruent; points on a perpendicular bisector of a line segment are exactly those equidistant from the segment’s endpoints.
problem_activity: Find two values where
$${\mathrm{cos}x=-1}$$
.
What is
$${\mathrm{arccos}(-1)}$$
?


aligns: no