Find the area of a triangle with base 16 and height 10.
Unit 10 · Lesson 02 · Area, Measurement & Geometric Probability
Area of Regular Polygons and Composite Figures
Use apothem, perimeter, central angle, and decomposition into congruent triangles.
Learning intention
Use accurate diagrams, definitions, measurements, and reasoning to use apothem, perimeter, central angle, and decomposition into congruent triangles.
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
Decompose figures, manage units and precision, and use area ratios to model probability and optimize design constraints.
A=\frac12 bhA=\frac12 aPA=\sqrt{s(s-a)(s-b)(s-c)}P(E)=\frac{\text{favorable measure}}{\text{total measure}}
Worked reasoning
Process: Decompose or select a suitable formula, keep exact values until the final step, attach square units, and assess whether rounding is reasonable.
- 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: Area uses square units and scales by the square of a linear scale factor; it does not scale linearly.
Khaemenes Academy · Open-Age Geometry
Area of Regular Polygons and Composite Figures
Find, construct, prove, or explain each result. Show reasoning and label units.
Name
Date
A regular polygon has apothem 5 and perimeter 36. Find its area.
Find the area of a triangle with side lengths 5, 5, and 6.
A figure is dilated by scale factor 3. By what factor does its area change?
A target occupies 18 cm² inside a 60 cm² board. Find the geometric probability of a uniform hit.
Why should exact π or radical form usually be kept until the final step?