TRIANGLES—GENERAL
In the figure above, the angles marked a° and b° are adjacent and supplementary, so a + b =
180 . Furthermore, the angles marked a° and 60° are vertical and equal, so a = 60.
78 . Parallel Lines and Transversals
A transversal across parallel lines forms four equal acute angles and four equal obtuse
angles, unless the transversal meets the lines at a right angle; then all eight angles are right
angles.
In the figure above, line 1 is parallel to line 2. Angles a, c, e, and g are obtuse, so they are all
equal. Angles b, d, f, and h are acute, so they are all equal.
Furthermore, any of the acute angles is supplementary to any of the obtuse angles. Angles a
and h are supplementary, as are b and e, c and f, and so on.
79 . Interior and Exterior Angles of a Triangle
The three angles of any triangle add up to 180 degrees.
In the figure above, x + 50 + 100 = 180, so x = 30.
An exterior angle of a triangle is equal to the sum of the remote interior angles.
