### Theory:

A triangle has three sides, three vertices, and three angles. A triangle with vertices $$A$$, $$B$$, and $$C$$ is denoted as $$△ABC$$.
Properties of a triangle:

The first property of a triangle: The sum of the three interior angles of a triangle is always $$180°$$.
$$∠A + ∠B + ∠C = 180°$$.

The second property of a triangle: The sum of the length of two sides of a triangle is always greater than the length of the third side of a triangle.
$$AB + BC > AC$$ or $$BC + AC > AB$$ or $$AB+ AC > BC$$.