Two triangles are congruent when they have the same shape and size. That means their corresponding sides are of equal length and their corresponding angles are of equal measure.

Two triangles are similar when they have the same shape. That means their corresponding angles are equal and that the length of corresponding sides are scaled by a common scale factor.

A ratio is a relationship between two numbers indicating how many times the first number contains the second. It is a way to compare quantities of the same kind by division. Ratios can be expressed in various forms, such as "a to b", "a:b", or as a fraction 

The factor a similar figure has been scaled up or down by. The scale factor is a number larger than 1 in the case of a scale-up, and a number between 0 and 1 in the case of a scale-down.

Mathagon

Definition

Scale Factor

AA Similarity Rule

SSS Similarity Rule

Why the SSS Rule Works

SAS Similarity Rule

Calculate the Length of a Missing Side 

Similar Triangles

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