Two triangles are congruent if two sides and the included angle
are congruent to the corresponding parts of another triangle.
, , and
. Therefore, .
The corresponding angles of congruent triangles are equal in
measure. Therefore, .
, , and form a straight
angle . Write an equation to show that the sum of the
angle measures is . Let represent
and , and solve for the value of .
Since , then is the same
perpendicular distance from as is from
. Therefore, is parallel to
and are alternate interior angles
formed by a line intersecting two parallel lines. Therefore, the
angles are equal in measure: