To see if 2 triangles are similar, we don't need to know that all three sides and all three angles. Usually, if we know two or three out of the six, it will be enough.

There are three theorems to find if two triangles are similar: AA, SAS and SSS:

AA - which stands for "angle, angle" and it states that if 2 triangles have two of their angles equal, then these 2 triangles are similar (Remember the sum of all interior angles of a triangle is always 180^o).

What this also means is that AA could also be called AAA, as when two angles are equal, all three angles must be equal.

SAS - which stands for "side, angle, side" and it states that if 1 triangle has

the ratio between two sides equal to the ratio between another two sides of another triangle, and that the included angles are equal between these 2 triangles, then these 2 triangles are similar.

SSS - which stands for "side, side, side" and it states that if 1 triangle has the ratio between all its 3 sides the same as the ratio between all 3 sides of another triangle, then these 2 triangles are similar.