Parallel lines cut by another line: These things are full of congruent and
supplementary angles. You could try to memorize which pairs of angles are
congruent and which pairs are supplementary, but why bother? The ones that
look supplementary are supplementary, and the ones that look congruent are
congruent. In some problems, you may need to extend the lines for it to look like
this:
Parallelogram: Opposite sides are parallel. Parallelograms have two pairs of
equal (or congruent) angles and four pairs of supplementary angles. In the
diagram, the ones that look equal are equal and the ones that look supplementary
are supplementary.
Triangles: Triangles are three-sided shapes whose interior angles add up to
180 degrees. One very, very heavily tested rule about triangles is the third side
rule. This states that the length of any side of a triangle has to be less than the
sum of the other two side lengths and greater than the difference of the other two