“What’s the point in proving theorems that have already been proved?” If your **geometry class** is anything like mine, then you have been asked to prove an existing theorem and you may have asked yourself the same question as above. Well there is the easy answer, because “I said so”. I agree, that is pretty lame and in reality being able to write a proof of an established theorem requires an ability to recall facts and to apply those facts and given information to arrive at a valid conclusion.

One such proof is of the **Converse of the Alternate Interior Angles Theorem**.

### Converse of the Alternate Interior Angles Theorem

If **two lines and a transversa**l form **alternate interior angles** that are congruent,

then the **two lines are parallel**.

One proof method of proof that I have stayed away from is the flow proof. The flow is too much work for the end product. It takes more time to complete the same problem. The image in this post took twice as much time to create than a simple image and writing a paragraph proof on my math blog. I came back to this section, because the a previous post on Parallel Lines is my top page receiving hits, so I wanted to do something more.

First off, the diagram is missing a label that is necessary for a paragraph proof of the converse of the alternate interior angles theorem. The line that is acting as the transversal of lines l and m will be called line t.

Given that line t is the transversal of lines l and m. By definition, Angle 3 and angle 1 are vertical angles and are congruent by the vertical angles theorem. It is given that angle 1 is congruent to angle 2. Applying the transitive property of congruence, it can be established that angle 2 is congruent to angle 3. It should be said that angle 2 and angle 3 are corresponding angles.Thus line l is parallel to line m because of the converse of the corresponding angles postulate. QED

Filed under: Converse of the Alternate Interior Angles Thm, Geometry, Proofs, Theorem Proof | Tagged: converse of alternate interior angles theorem, Geometry |

Sara, on September 6, 2012 at 7:05 am said:Hello! This question might be somewhat foolish but how would you know whether which step is next without adding any unneccesary steps? ( like the above example, how would you know if the first step is vertical angle theorem or definition of congruent angles? There’s just so many possibilities!)

-Sara

Mr. Pi, on September 12, 2012 at 6:11 am said:I am not following you. I did not use the definition of congruent angles in my proof.