What if you were presented with two angles that are on the same side of a transversal, but inside the lines? How would you describe these angles and what could you conclude about their measures? After completing this Concept, you'll be able to answer these questions using your knowledge of same side interior angles.
Watch This
CK12 Foundation: Chapter3SameSideInteriorAnglesA
Watch the portions of this video dealing with same side interior angles.
James Sousa: Angles and Transversals
James Sousa: Proof that Consecutive Interior Angles Are Supplementary
James Sousa: Proof of Consecutive Interior Angles Converse
Guidance
Same Side Interior Angles
are two angles that are on the same side of the transversal and on the interior of the two lines.
Same Side Interior Angles Theorem: If two parallel lines are cut by a transversal, then the same side interior angles are supplementary.
So, if
Converse of the Same Side Interior Angles Theorem: If two lines are cut by a transversal and the consecutive interior angles are supplementary, then the lines are parallel.
Example A
Using the picture above, list all the pairs of same side interior angles.
Same Side Interior Angles:
Example B
Find
Here,
This example shows why if two parallel lines are cut by a transversal, the same side interior angles are supplementary.
Example C
Find the measure of
The given angles are same side interior angles. The lines are parallel, therefore the angles add up to
Watch this video for help with the Examples above.
CK12 Foundation: Chapter3SameSideInteriorAnglesB
Vocabulary
Same Side Interior Angles
are two angles that are on the same side of the transversal and on the interior of the two lines. Two angles are
supplementary
if they add to
Guided Practice
1. Is
2. Find the value of
3. Find the value of
Answers:
1. These are Same Side Interior Angles. So, if they add up to
2. The given angles are same side interior angles. Because the lines are parallel, the angles add up to
3. These are same side interior angles so set up an equation and solve for
Interactive Practice
Practice
For questions 12, determine if each angle pair below is congruent, supplementary or neither.

∠5 and∠8 
∠2 and∠3  Are the lines below parallel? Justify your answer.
In 45, use the given information to determine which lines are parallel. If there are none, write none. Consider each question individually.

∠AFD and∠BDF are supplementary 
∠DIJ and∠FJI are supplementary
For 611, what does the value of

m∠3=(3x+25)∘ andm∠5=(4x−55)∘ 
m∠4=(2x+15)∘ andm∠6=(3x−5)∘ 
m∠3=(x+17)∘ andm∠5=(3x−5)∘ 
m∠4=(3x+12)∘ andm∠6=(4x−1)∘ 
m∠3=(2x+14)∘ andm∠5=(3x−2)∘ 
m∠4=(5x+16)∘ andm∠6=(7x−4)∘
For 1213, determine whether the statement is true or false.
 Same side interior angles are on the same side of the transversal.
 Same side interior angles are congruent when lines are parallel.
For questions 1415, use the picture below.

What is the same side interior angle with
∠3 ?  Are the lines parallel? Explain.