Two geometric figures are said to be similar if they have the same shape regardless of the size differences between them. For triangles, we have the following criteria that help us determine when they are similar:

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Angle-Angle Criterion (AA)

Two triangles are similar if they have two equal angles.

Criterio 1 ALA triángulo grande representación gráfica
Criterio 1 ALA triángulo pequeño representación gráfica

Side-Side-Side Criterion (SSS)

Two triangles are similar if they have proportional sides.

Criterio 2 LLL triángulo grande representación gráfica
Criterio 2 LLL triángulo pequeño representación gráfica

Side-Angle-Side Criterion (SAS)

Two triangles are similar if they have two proportional sides and the angle between them is equal.

Criterio 3 LAL triángulo grande representación gráfica
Criterio 3 LAL triángulo pequeño representación gráfica

Similarity of Triangles

Similarity Criteria for Right Triangles

Two right triangles are similar if they have one equal acute angle.

Two right triangles are similar if they have the two legs proportional.


Two right triangles are similar if the hypotenuse and one leg are proportional.

Exercises on Triangle Similarity

Determine whether the following triangles are similar:

1

Ejercicio 1 de triángulo pequeño criterio LLL representación gráfica
Ejercicio 1 de triángulo grande criterio LLL representación gráfica

Solution:

They are similar because they have their 3 sides proportional.

2

Ejercicio 2 de triángulo grande criterio ALA representación gráfica
Ejercicio 2 de triángulo pequeño criterio ALA representación gráfica

Solution:

They are similar because they have two equal angles.

3

Ejercicio 3 de triángulo pequeño criterio LAL representación gráfica
Ejercicio 3 de triángulo grande criterio LAL representación gráfica

Solution:

    and

    and 

They are similar because they have two proportional sides and one equal angle.

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Agostina Babbo

Agostina Babbo is an English and Italian to Spanish translator and writer, specializing in product localization, legal content for tech, and team sports—particularly handball and e-sports. With a degree in Public Translation from the University of Buenos Aires and a Master's in Translation and New Technologies from ISTRAD/Universidad de Madrid, she brings both linguistic expertise and technical insight to her work.