Geometric Proofs
Geometric proofs use logical reasoning to demonstrate that geometric statements must be true. We use known angle and shape properties to build step-by-step arguments.
What You Need to Know
Key Concept Diagram
Angles on a straight line sum to 180 degrees (supplementary angles)
Angles in a triangle sum to 180 degrees
Vertically opposite angles are equal
Co-interior angles between parallel lines sum to 180 degrees; alternate angles are equal
Key Vocabulary
Theorem
A proven mathematical statement that can be used in further proofs
Supplementary
Two angles that add to 180 degrees
Vertically opposite angles
Equal angles formed when two lines cross
Transversal
A line that crosses two or more parallel lines
Knowledge Check
Select the correct answer for each question. Click "Check Answer" to see if you are right.
Question 1
Two lines cross forming angles of x and 130 degrees. What is x?
Question 2
A triangle has angles 65 degrees and 75 degrees. What is the third angle?
Question 3
Alternate angles between parallel lines are:
Key Concepts Summary
- ●Angles on a straight line sum to 180 degrees (supplementary angles)
- ●Angles in a triangle sum to 180 degrees
- ●Vertically opposite angles are equal
- ●Co-interior angles between parallel lines sum to 180 degrees; alternate angles are equal