A triangle is a shape with three lines. The internal angles of all triangles when added together equal 180 degrees. If you know two angles of a triangle, you can determine the third angle by deducting the two angles from 180. For example:

60 + 90 + ? = 180
150 + ? = 180
? = 180 - 150
? = 30 degrees

There are several classifications of triangles according to their sides.

Scalene Triangle

None of the sides are congruent (equal to each other).

Isosceles Triangle

Two of the sides are congruent.

Equilateral Triangle

All three sides are congruent.

There are also classifications of triangles according to their angles.

Acute Triangle

All three angles are acute (less than 90 degrees).

Right Triangle

One of the angles is 90 degrees (a right angle).

Obtuse Triangle

One of the angles is obtuse (greater than 90 degrees).

Similar Triangles

Triangles may have the same angles, but different lengths for their sides. This will result in a similar triangle with different proportions. The lengths of a pair of corresponding sides have the same ratio as the lengths of any other pair of corresponding sides. For example:

DE : AB = DF : AC = EF : BC

12:6 = 10:5 = 6:3 (review proportions if required)

All corresponding angles would be equal with similar triangles. Angle D = Angle A, Angle E = Angle B, and Angle C = Angle F.