Learning Goal:

• To be able to apply the rules of congruence to identify congruent figures

In Geometry, congruent figures are two figures (2D) that have the same shape and size. If two figures are congruent, one of the figures can be rotated, reflected and/or translated (shifted up, down, left and/or right) to match the other figure exactly. 

If triangle DEF (ΔDEF) is congruent to triangle RST (ΔRST), we write ΔDEF ≅ΔRST (triangle DEF is congruent to triangle RST). This is called a congruence statement. 

Corresponding sides and angles refer to the pairs of matching sides or angles that are in the same position in two different shapes. 

Corresponding sides

DE = RS

EF = ST

DF = RT 

Corresponding angles

∠DEF = ∠RST

∠EFD = ∠STR

∠EDF = ∠SRT

Testing for congruence

There are four congruence tests used for triangles. If two triangles satisfy one of the following tests, the two triangles are proved to be congruent. 

SSS: side-side-side

Two triangles are congruent if all corresponding sides are equal. 

SAS: side-angle-side

Two triangles are congruent if two pairs of corresponding sides and the angle included between them is equal. 

ASA: angle-side-angle

Two triangles are congruent if two pairs of corresponding angles and a pair of corresponding sides are equal. 

RHS: right angle-hypotenuse-side

Two right-angled triangles are congruent if the hypotenuses and one pair of corresponding sides are equal.