Geometry help
Free Geometry help with angles, proofs, triangles, similarity, circles, area, and volume. Learn each idea through explanations, figures, and worked examples.
Where to start
Begin with the language of points, lines, and angles. Angle relationships support triangle work and proofs; congruence and similarity support later work with right triangles and circles. Use the lesson list to return to a theorem you need.
Mark the information given in a figure before choosing a theorem. Do not assume lines are parallel or angles are equal just because they look that way. For measurement problems, include units and check whether the answer is a length, area, or volume.
Geometry
96 lessonsTriangles & the Pythagorean Theorem
- 9.1The Pythagorean Theorem
- 9.2Geometric Mean & the Altitude-on-Hypotenuse Relationships
- 9.3The Pythagorean Theorem & Its Converse
- 9.4Special Right Triangles: 45-45-90
- 9.5Special Right Triangles: 30-60-90
- 9.6Sine, Cosine & Tangent Ratios
- 9.7Solving for Missing Sides with Trig
- 9.8Inverse Trig: Finding Missing Angles
- 9.9Angles of Elevation & Depression
Tools of Geometry
Angles
Proof & Parallel Lines
- 3.1Inductive vs. Deductive Reasoning
- 3.2Logic: Negation, Conjunction, Disjunction & Truth Tables
- 3.3Intro to Reasoning: Conditional Statements & Logic
- 3.4Converse, Inverse, Contrapositive & Biconditionals
- 3.5Two-Column, Paragraph & Flow Proofs
- 3.6Algebraic Proof & Justifying Each Step
- 3.7Proving Segment Relationships
- 3.8Proving Angle Relationships
- 3.9Precise Definitions: Perpendicular & Parallel Lines
- 3.10Parallel Lines Cut by a Transversal
- 3.11Slope Criteria for Parallel & Perpendicular Lines
- 3.12Proving Lines Parallel
- 3.13Perpendiculars & Distance
Transformations & Symmetry
Congruent Triangles
- 5.1Angles of Triangles
- 5.2Congruent Figures & Corresponding Parts (CPCTC)
- 5.3SSS & SAS Triangle Congruence
- 5.4ASA & AAS Triangle Congruence
- 5.5HL Congruence for Right Triangles
- 5.6Triangle Congruence Proofs
- 5.7Isosceles & Equilateral Triangle Theorems
- 5.8Classifying Triangles & Quadrilaterals by Coordinates
- 5.9Coordinate Proofs
Triangle Relationships
Quadrilaterals
Similarity
Circles
- 10.1Circle Vocabulary: Center, Radius, Chord, Secant, Tangent
- 10.2Central Angles & Arc Measure
- 10.3Circumference & Arc Length
- 10.4Arcs & Chords
- 10.5Inscribed Angles & the Inscribed-Angle Theorem
- 10.6Tangent Lines & the Two-Tangent Theorem
- 10.7Tangents, Secants & Chords: Angle Relationships
- 10.8Segment-Length Relationships in Circles
- 10.9The Equation of a Circle
Area & Volume
- 11.1Area & Perimeter of Triangles & Quadrilaterals
- 11.2Perimeter & Area on the Coordinate Plane
- 11.3Area of Regular Polygons
- 11.4Area of Circles
- 11.5Sectors & Segment Area
- 11.6Composite & Shaded-Region Area
- 11.7Solids, Nets & Cross-Sections
- 11.8Solids of Revolution
- 11.9Surface Area of Prisms & Cylinders
- 11.10Surface Area of Pyramids & Cones
- 11.11Volume of Prisms & Cylinders
- 11.12Volume of Pyramids & Cones
- 11.13Surface Area & Volume of Spheres
- 11.14Scale Factor: Perimeter, Area & Volume
- 11.15The Effect of Scale Factor on Volume
- 11.16Modeling with Geometry: Density & Design
Free Geometry worksheets
Use these printable worksheets for extra practice, with an answer key included.
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