Exercises: Prove Theorems About Parallelograms

Work through each section in order. Show all proof steps and justifications where indicated.

Grade 9·21 problems·~30 min·Common Core Math - HS Geometry·standard·hsg-co-c-11

Work through problems with immediate feedback

Warm-Up: Review What You Know

These problems review skills from earlier in the course that you will need for parallelogram proofs.

Lines $\ell$ and $m$ are parallel, and line $t$ is a transversal crossing both. Which statement about the alternate interior angles is true?

They are supplementary (they sum to $180^\circ$ ).

They are congruent.

They are complementary (they sum to $90^\circ$ ).

They are vertical angles.