Similar TrianglesSimilar triangles have the same shape, but the size may be different. Remember "≅" means "is congruent to" and "~" is "similar to". Examples
Two triangles are similar if:
Notice the corresponding angles for the two triangles in the applet are the same. The corresponding sides lengths are the same only when the scale factor slider is set at 1.0. Study the side lengths closely and you will find that the corresponding sides are proportional.
If you know triangles are similar, you can use the proportion of corresponding sides to help determine an unknown dimension. Study the object below. You can change the triangles by dragging on the slider or dragging vertex A or B. The proportions of the corresponding pairs of sides changes as the scale slider position changes. Each time you stop dragging, look at the proportions. If one of the sides was unknown you could use two pairs of corresponding sides to calculate the missing dimension.


Congruent triangles are a special type of similar triangles. Congruent triangles have the same shape (similar triangles) and size. Increase/decrease <A, <B and <C by clicking and dragging the vertices of the left triangle below. Notice the corresponding angles (<D <E and <F) remain congruent. Since the sum of three angles must be 180^{o}, the third pair of corresponding angles must also be congruent when the first two pairs of corresponding angles are congruent.
In the applet above: <A = <D, <B = <E, <C = <F Notice the size and shape of the new pairs of triangles remains the same. The patterns you may have observed and need to know for congruent triangles are displayed below. Remember "≅" means "is congruent to". Two triangles are congruent if:
The factor for congruent triangles is 1. Remember this fact for future problem solving activities.
If all 3 pairs of corresponding sides in two triangles are the same, then the triangles are congruent. Move the corners A, B or C of the triangle above. Watch how ΔDEF changes. Since the 3 pairs of corresponding sides of the two triangles are equal each time you manipulate the applet, the SSS (Side, Side, Side) Congruence Relation proves each pair of triangles are congruent.

Triangle Congruence Relations


Always form congruent triangles 
May not form congruent triangles: 
SSS
If three sides of one triangle are congruent to three sides of a second triangle, the two triangles are congruent.
ASA If two angles and the included side of one triangle are congruent to two angles and the included side of another triangle, the triangles are congruent. SAS
If two sides and the included angle are congruent to two sides and the included angle of a second triangle, the two triangles are congruent. AAS
If two angles and a non included side of one triangle are congruent to two angles and the corresponding nonincluded side of another triangle, the two triangles are congruent. HypS
If the hypotenuse and the leg of one right triangle are congruent to the corresponding parts of the second right triangle, the two triangles are congruent 
SSA
Two triangles with two sides and a nonincluded angle equal may or may not be congruent. AAA If two angles on one triangle are equal, respectively, to two angles on another triangle, then the triangles are similar, but not necessarily congruent. 