The external angle of a triangle is greater than either of the opposite internal angles. In the words of Euclid: In any triangle, if one of the sides be produced, the The exterior angle is greater than either interior and opposite angles.

Triangle Exterior Angle Theorem Remember that the two non-adjacent interior angles opposite the exterior angle are sometimes referred to as remote interior angles. An exterior angle of a triangle is equal to the sum of the two opposite interior angles.

An exterior angle of a triangle is formed when one side of a triangle is extended. The nonstraight angle (the one that is not just the extension of the side) outside the triangle, but adjacent to an interior angle, is an exterior angle of the triangle (Figure 1 ). In Figure 1 , ∠ BCD is an exterior angle of Δ ABC.

Because an exterior angle is equal to the sum of the opposite interior angles, it follows that it must be larger than either one of them. Stated more formally: Theorem: An exterior angle of a triangle is always larger then either opposite interior angle.

With each pair of alternate interior angles, both angles are inside the parallel lines and on opposite (alternate) sides of the transversal. There are thus two pairs of these angles.

Alternate angles are angles that are in opposite positions relative to a transversal intersecting two lines. If the alternate angles are between the two lines intersected by the transversal, they are called alternate interior angles.

When a transversal intersects two lines, the two lines are parallel if and only if interior angles on the same side of the transversal and exterior angles on the same side of the transversal are supplementary (sum to 180°).

1. Complement of 70 is 90-70= 20 degrees.