
Ontario Curriculum Expectation:
6.E2.3 use the properties of supplementary angles, complementary angles, opposite angles, and interior and exterior angles to solve for unknown angle measures.
Two intersecting lines create a pair of opposite angles. Opposite angles are equal.
In the figure below, two straight lines intersect to create a matching pair of opposite angles.
∠a and ∠b are opposite angles, so they have the same measurement.