SAT Math: Similar Triangles and Proportional Sides
33+ practice questions in Praczo
The concept, explained
- 1
Two triangles are similar if their corresponding angles are equal. Corresponding sides are proportional.
- 2
AA (Angle-Angle) similarity: if two angles of one triangle equal two angles of another, the triangles are similar.
- 3
Set up proportions with corresponding sides in the same position: short/short = long/long.
- 4
When a line is drawn parallel to one side of a triangle, it creates a smaller similar triangle.
- 5
Scale factor k: if triangles are similar with ratio k, areas scale by k² and perimeters scale by k.
- ✗ Matching non-corresponding sides in the proportion — always match the sides opposite equal angles.
- ✗ Forgetting that similarity gives ratios, not equality. You must use the scale factor to find the actual side lengths.
SAT-style practice
Triangle ABC is similar to Triangle DEF. AB = 6, BC = 9, and DE = 4. What is the length of EF?
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