Similar & Congruent Figures
Similar figures have equal angles and proportional sides.
The rules
- Set up a proportion between matching sides.
- If the scale factor is k, areas scale by k² and volumes by k³.
- Congruent means identical in shape and size (SSS, SAS, ASA).
- Name corresponding parts in order.
Watch out: Using the scale factor for area instead of its square.
Worked example
Two similar triangles have sides in the ratio 2:3. If the small triangle has area 20, what is the area of the large one?
- Area ratio is 4:9.
- 20 · 9/4 = 45.
Answer: 45
Try it yourself
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