Congruence: conditions for congruent triangles
In a nutshell
Congruent shapes are shapes that are exactly the same apart from rotational/mirror differences. There are four main conditions for two triangles to be congruent.
Congruency
Two shapes are congruent if they have the same side lengths and the same angles. It doesn't matter if the shapes are rotated slightly differently or if they're mirrored.
Congruency conditions
There are four ways to prove that two triangles are congruent.
name | description | example |
SSS | All three sides of the triangle are the same | |
SAS | Two sides and the angle between them are the same | |
AS | Two angles and a corresponding side are the same | |
RHS | Both triangles have a right-angle, the same hypotenuse and other common side | |
Example 1
Are the two triangles shown congruent to one another?
At first glance, the two triangles look to be congruent due to the ASA rule - both triangles have angles of 40∘ and 80∘ and they both have a side of 2.2cm.
However, upon closer inspection, this is not a case of ASA. This is because the two sides aren't corresponding.
The triangle to the left has a side length of 2.2cm that is between the two angles of 40∘ and 80∘.
The triangle to the right has a side length of 2.2cm that is not between the two angles of 40∘ and 80∘, so it's not the corresponding sides that are equal.
The triangles are NOT congruent.