Lines & Angles
Complementary, supplementary, vertical, and triangle angles, and transversals.
This page is a free outline of what the Lines & Angles lesson covers — it is not the lesson itself. The interactive lesson, practice, quizzes, and AI Tutor are inside the app.
This lesson covers
- Angles in Triangles The three interior angles of any triangle always add up to 180°, so if you know two, subtract their sum from 180° to find the third.
- Complementary Angles Complementary angles are two angles that add up to 90° — a “corner.” If one is 35°, its complement is 55°.
- Parallel and Perpendicular Lines Parallel lines have the same slope and never meet; perpendicular lines cross at a right angle and have slopes that multiply to −1 (negative reciprocals).
- Parallel Lines and Transversals When a transversal crosses two parallel lines it makes 8 angles: corresponding and alternate-interior angles are equal, and co-interior (same-side) angles add to 180°.
- Supplementary Angles Supplementary angles are two angles that add up to 180° — a straight line. If one is 110°, its supplement is 70°.
- Types of Angles Angles are named by their size: acute (less than 90°), right (exactly 90°, marked with a small square), obtuse (between 90° and 180°), and straight (exactly 180°, a flat line).
- Vertical Angles Vertical angles are the opposite angles formed where two lines cross, making an X — the angles across from each other are always equal.
Hover or tap a dotted ⓘ concept for a quick definition.
Inside the app, Lines & Angles comes with:
Read the explanation here for free — then practice it for real, with everything you need in one place.
24/7 AI Tutor — Ask anything and get a step-by-step explanation when you’re stuck — it teaches the concept, not just the answer. Every answer verified by three independent AIs.
- Practice sets — Open-book practice questions at your level, easiest-first — as many as you want, never scored.
- Quizzes — Scored quizzes on the whole lesson, so you know when you’re ready to move on.
- Skills Check — A concept-by-concept map of what you’ve mastered and what to focus on next.
- Worked examples — Tap the Brain Button on any example for alternative step-by-step explanations.
- Glossary — Plain-English definitions of every key term in the lesson.
- Formula sheet — The GED® & HiSET® formula sheets, on screen while you practice.
- Practice calculator — An on-screen calculator to build your calculator skills before test day.
- Math-fact flashcards — Quick drills for addition, multiplication, perfect squares, and square roots.
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