Geometry and TrigonometryHardUnknown side in similar triangles with a decimal answer
Geometry and Trigonometry practice question
Triangles ABC and DEF are similar, with A, B, C corresponding to D, E, F. AB = 6, DE = 15 and DF = 9. What is the length of AC?
Answer: 3.6 or 18/5
The scale factor from DEF to ABC is AB/DE = 6/15 = 2/5. AC corresponds to DF: AC = 9 × 2/5 = 3.6.
Common wrong answers
- If you answered 0
- 0 comes from 9 − 9 or a dropped calculation; corresponding sides scale by 2/5.
- If you answered 9
- 9 is DF itself; AC is the corresponding side of the smaller triangle.
- If you answered 22.5
- 22.5 scales the wrong way (9 × 15/6); ABC is the SMALLER triangle.
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