Angles in Triangles and Parallel Lines Worksheets

Sample problems

1. Name the three pairs of angles made when parallel lines are cut by a transversal, and say what is true of each.
2. A transversal makes 40° with two parallel lines. Give the alternate, corresponding and co-interior angles.
3. A triangle has angles 35° and 85°. Find the third angle.
4. Find the exterior angle at each vertex of a triangle with angles 50°, 60° and 70°. What do the three exterior angles add to?

Common mistakes to watch for

• Gave 65° for all three. Two of the three named pairs ARE equal, so the pattern fits two cases out of three — which is exactly why the third is the one that gets missed.
• Said “because co-interior angles add to 180” and stopped. That is the fact, not the argument, and the question asks for the argument — which is also what makes the fact recoverable when it is forgotten. The 130-instead-of-180 observation is the strongest form of the answer.
• Gave the exterior angle as 70°, taking it to be the interior angle at that vertex, or as 180°, adding all three. Checking against 50 + 60 catches both instantly.

The answer key explains each one, so you can tell what a wrong answer means.