Proving Theorems about Angles
Aligned to G-CO.C.9 — Common Core State Standards for Mathematics.
What this lesson teaches
Geometric proofs use given facts, definitions, and logical steps to reach a conclusion. Angle relationships (vertical, supplementary, parallel-line angles) are common tools.
Worked example
Vertical angles are equal: when two lines cross, the opposite angles are congruent because each pairs with the same adjacent angle to make 180°.
Practice questions
- If two angles are supplementary and one is 110°, find the other.
- Why are vertical angles always equal?
- Two parallel lines are cut by a transversal — name a pair of equal angles.
Watch the lesson
Every lesson comes with a video taught in English and Spanish — the same video the QR code in the printed workbook opens.
▶ Watch this lessonEn español
Demostración de teoremas sobre ángulos
Las demostraciones geométricas usan datos dados, definiciones y pasos lógicos para llegar a una conclusión. Las relaciones entre ángulos (ángulos verticales, suplementarios y ángulos formados por líneas paralelas) son herramientas comunes.
Ejemplo: Los ángulos verticales son iguales: cuando dos líneas se cruzan, los ángulos opuestos son congruentes porque cada uno forma un par con el mismo ángulo adyacente para sumar 180°.
More Geometry lessons
Teach this lesson today
An AI tutor that guides with questions instead of just giving answers — all K–12 core subjects, every child in the family.
Start free — no card required