A triangle has angles 60° and 75°. Find the third angle.
Unit 03 · Lesson 01 · Triangle Congruence & Proof
Triangle Classification & Angle Relationships
Classify by sides and angles, use the triangle angle sum, and connect exterior angles to remote interior angles.
Learning intention
Use accurate diagrams, definitions, measurements, and reasoning to classify by sides and angles, use the triangle angle sum, and connect exterior angles to remote interior angles.
Success evidence
- Label the figure and units before calculating.
- Select and justify the governing definition, theorem, relationship, or construction.
- Show enough work that another learner can audit the conclusion.
- Check the result against the diagram, scale, and stated constraints.
Visual model
Identify sufficient evidence for triangle congruence and organize theorem-based proofs about corresponding parts and centers.
m\angle A+m\angle B+m\angle C=180^\circa+b>cSSS,\ SAS,\ ASA,\ AAS,\ HL
Worked reasoning
Process: Match vertices in a consistent correspondence, mark the known evidence, select the valid congruence theorem, then use CPCTC only after congruence is established.
- Read the figure and rewrite all given information with correct notation.
- Identify the exact relationship that connects the known and unknown quantities.
- Substitute, construct, or prove one justified step at a time.
- State the result with units and a sentence explaining what it means.
Misconception check: AAA proves similarity, not congruence; SSA is not a general congruence theorem.
Khaemenes Academy · Open-Age Geometry
Triangle Classification & Angle Relationships
Find, construct, prove, or explain each result. Show reasoning and label units.
Name
Date
Two sides are 11 and 11. State the integer range for the third side x.
Which congruence theorem uses two sides and the included angle?
After proving ΔABC≅ΔDEF, which angle corresponds to ∠B?
Where do the perpendicular bisectors of a triangle meet?
Why does AAA not prove triangle congruence?