Given: e parallel f and g is a transversal prove: angle 1 is-congruent-to angle 8 horizontal and parallel lines e and f are cut by transversal g. on line e where it intersects line g, 4 angles are formed. labeled clockwise, from uppercase left, the angles are: 1, 2, 4, 3. on line f where it intersects line g, 4 angles are formed. labeled clockwise, from uppercase left, the angles are: 5, 6, 8, 7. given that e parallel f and g is a transversal, we know that angle 4 is-congruent-to angle 5 by the alternate interior angles theorem. we also know that angle 1 is-congruent-to angle 4 and angle 5 is-congruent-to angle 8 by the therefore, angle 1 is-congruent-to angle 8 by the substitution property. corresponding angles theorem alternate interior angles theorem vertical angles theorem alternate exterior angles theorem