
2-8 Areas, heights and the line parallel to one side
Point 8 of 10 — Unit 2: Similarity of Polygons and Triangles, Areas and Circle TheoremsQuestion bank: 0 questions (0 with pictures)
08 2-8 Areas, heights and the line parallel to one side
- The ratio of the areas of two similar polygons equals the square of the scale factor: A₁/A₂ = k².
- The ratio of perimeters equals k itself; so from an area ratio you get k by taking the square root, and from k you get an unknown height or perimeter.
A line parallel to one side of a triangle
- If DE ∥ BC in triangle ABC, then △ADE ~ △ABC, and the trapezoid DBCE has area = area(△ABC) − area(△ADE).
- Hence area(△ADE)/area(DBCE) = AD² : (AB² − AD²) — a standard exam form that asks for a ratio to the trapezoid, not to the whole triangle.
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