Angle Relationships
Triangles sum to 180°, and pairs of angles come in fixed families.
Concept
The sums to memorize
The three interior angles of any triangle add to 180°. A quadrilateral adds to 360°.
Complementary angles add to 90°, supplementary angles add to 180°, and vertical angles (across an X) are equal.
∠A + ∠B + ∠C = 180°