Properties of a triangle are
0. Any triangle is in a single plane.
1 the relationships between the sizes of the sides.
a + b > C > | a - b |
2 the relation ships between the interior angles
Sum of A, B, C is 180 degrees.
3 the relationships between the angles and the sides
4. the relationship between interior and exterior angles.
5. relationship between area, perimeter, center of gravity, circumcircle, incircle etc. and the sides.