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Similar Triangle Criteria Calculator

Online similar triangles calculator. Enter three corresponding sides of two similar triangles to find the fourth by proportionality—ideal for triangle similarity.

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b2 = b1 × (a2/a1)
∼
Side in triangle 1
Corresponding side in triangle 1
Corresponding side in triangle 2

📖 Tutorial | Similar Triangles

1. Definition
Triangles with equal angles and proportional sides are similar.
2. Formula & Notation
kScale factor
a1, b1Triangle 1 sides
a2, b2Triangle 2 sides
3. Properties & Laws
  • Equal angles, proportional sides;
  • Perimeter ratio = factor;
  • Area ratio = factor squared;
  • Congruence is factor 1.
4. Steps
  1. Enter three corresponding sides;
  2. Click "Find b2";
  3. Or click "Load Example".
5. Worked Examples
Example: a1=3, b1=5, a2=6.
Solution: factor 2, b2 = 10.
6. Common Pitfalls
Sides must correspond;
Area ratio is squared;
Mind the order.

❓ FAQ | Similar Triangles

How are they tested?
Two equal angles, proportional sides with included angle, or three proportional.
Scale factor?
Ratio of corresponding sides.
Perimeter and area?
Perimeter factor; area factor squared.
3,5,6 gives b2?
10.
Is congruence similarity?
Yes, factor 1.
Corresponding heights?
Equal to the factor.