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Same Side Interior Angles

Angles on the same side of a transversal and inside the lines it intersects.

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Co-Interior Angles

Learning Goal

By the end of this lesson you will be able to . . . identify co-interior angles and use your knowledge to find missing angle measures.

What if you were presented with two angles that are on the same side of the transversal and on the interior of the two 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 and apply same side (co-interior) interior angle theorems to find the measure of unknown angles.

Watch This

CK-12 Same Side Interior Angles

Watch the portions of this video dealing with same side interior angles (co-interior angles).

James Sousa: Angles and Transversals

Then watch this video.

James Sousa: Proof that Consecutive Interior Angles Are Supplementary

Finally, watch this video.

James Sousa: Proof of Consecutive Interior Angles Converse


Same side interior angles or Co-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 or Co-interior Angle Theorem: If two parallel lines are cut by a transversal, then the same side interior angles are supplementary.

If line "l" is parallel to "m", then angle 1 and angle 2 add up to 1800


Example A

Find \begin{align*}z\end{align*}.

\begin{align*}z+116^\circ=180^\circ\end{align*} so \begin{align*}z = 64^\circ\end{align*} by Same Side Interior Angles Theorem.

Example B

Is "l" parallel to "m"? How do you know?

These angles are Co-interior Angles. So, if they add up to \begin{align*}180^\circ\end{align*}, then l is parallel to m.

\begin{align*}130^\circ + 67^\circ = 197^\circ\end{align*}, therefore the lines are not parallel.

Example C

Give two examples of co-interior angles in the diagram below:

There are MANY examples of co-interior angles in the diagram. Two are \begin{align*}\angle 6\end{align*} and \begin{align*} \angle 10\end{align*}, and \begin{align*}\angle 8\end{align*} and \begin{align*}\angle 12\end{align*}.

CK-12 Same Side Interior Angles

Guided Practice

1. Find the value of \begin{align*}x\end{align*}.

2. Find the value of \begin{align*}y\end{align*}.

3. Find the value of \begin{align*}x\end{align*} if \begin{align*}m\angle 3 = (3x +12)^\circ\end{align*} and \begin{align*}\ m\angle 5 = (5x + 8)^\circ\end{align*}.


1. The given angles are same side interior angles. Because the lines are parallel, the angles add up to \begin{align*}180^\circ\end{align*}.

\begin{align*}(2x+43)^\circ + (2x-3)^\circ & = 180^\circ\\ (4x+40)^\circ & = 180^\circ\\ 4x & = 140\\ x & =35\end{align*}

2. \begin{align*}y\end{align*} is a same side interior angle with the marked right angle. This means that \begin{align*}90^\circ +y=180\end{align*} so \begin{align*}y=90\end{align*}.

3. These are same side interior angles so set up an equation and solve for \begin{align*}x\end{align*}. Remember that same side interior angles add up to \begin{align*}180^\circ\end{align*}.

\begin{align*}(3x+12)^\circ+(5x+8)^\circ&=180^\circ\\ (8x+20)^\circ&=180^\circ\\ 8x&=160\\x&=20\end{align*}


For questions 1-2, determine if each angle pair below is congruent, supplementary or neither.

  1. \begin{align*}\angle 5\end{align*} and \begin{align*}\angle 8\end{align*}
  2. \begin{align*}\angle 2\end{align*} and \begin{align*}\angle 3\end{align*}
  3. Are the lines below parallel? Justify your answer.

In 4-5, use the given information to determine which lines are parallel. If there are none, write none. Consider each question individually.

  1. \begin{align*}\angle AFD\end{align*} and \begin{align*}\angle BDF\end{align*} are supplementary
  2. \begin{align*}\angle DIJ\end{align*} and \begin{align*}\angle FJI\end{align*} are supplementary

For 6-8, what does the value of \begin{align*}x\end{align*} have to be to make the lines parallel?

  1. \begin{align*}m\angle 3 = (3x+25)^\circ\end{align*} and \begin{align*}m\angle 5 = (4x-55)^\circ\end{align*}
  2. \begin{align*}m\angle 4 = (2x+15)^\circ\end{align*} and \begin{align*}m\angle 6 = (3x-5)^\circ\end{align*}
  3. \begin{align*}m\angle 3 = (x+17)^\circ\end{align*} and \begin{align*}m\angle 5 = (3x-5)^\circ\end{align*}

For 9-10, determine whether the statement is true or false.

  1. Same side interior angles are on the same side of the transversal.
  2. Same side interior angles are congruent when lines are parallel.

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