Chapter 8: Similar Triangles
5 lessons
Similar Triangles covers the Basic Proportionality Theorem (a line parallel to one side of a triangle divides the other two proportionally), the AA/SAS/SSS similarity criteria, how similar triangles' areas relate to their side lengths, and the Pythagorean theorem — proved here using similarity rather than assumed.
This chapter deliberately contrasts with Class 9's Triangles — similarity (same shape) instead of congruence (same shape and size), and the ratio of similar triangles' areas equals the square of their side ratio, not the side ratio itself — a detail worth double-checking. Matching up corresponding sides incorrectly when writing a similarity ratio is the most common error, and it silently invalidates everything calculated afterward.