Same Side Interior Angles
Same side interior angles are two angles that are on the interior of (between) the two lines and specifically on the same side of the transversal. The sameside interior angles sum up to 180 degrees. When two parallel lines are intersected by a transversal line they formed 4 interior angles. The 2 nonadjacent interior angles that are on the same side of the transversal are supplementary.
1.  What are Same Side Interior Angles? 
2.  Same Side Interior Angles Theorem 
3.  Converse of Same Side Interior Angles Theorem 
4.  FAQs on Same Side Interior Angles 
What are Same Side Interior Angles?
When two parallel lines are intersected by a transversal, 8 angles are formed. The same side interior angles are the pair of nonadjacent interior angles that lie on the same side of the transversal.
So the same side interior angles:
 have no common vertices or have different vertices
 lie between two lines
 and formed on the same side of the transversal
The "same side interior angles" are also known as "cointerior angles."
The 8 angles thus formed are classified into different types of angles listed below:
 Alternate Interior Angles
 Corresponding Angles
 Alternate Exterior Angles
 Same Side Interior Angles or cointerior angles
In the given figure, line AB  CD and line l is the transversal.
From the "Same Side Interior Angles  Definition," the pairs of same side interior angles in the above figure are:
 4 and 5
 3 and 6
Same Side Interior Angles Theorem
Let us consider the above figure. In the above figure, lines AB and CD are parallel and L is the transversal. We just read that the pairs of the same side interior angles in the above figure are:
 ∠4 and ∠5
 ∠3 and ∠6
The relation between the same side interior angles is determined by the same side interior angle theorem.
The theorem for the "same side interior angle theorem" states: If a transversal intersects two parallel lines, each pair of sameside interior angles are supplementary (their sum is 180°).
Same Side Interior Angles Theorem Proof
Referring the above figure once again:
∠4 = ∠8, and ∠3 = ∠7 [corresponding angles are equal].
∠5 + ∠8 = 180° and ∠6 + ∠7 = 180° [ linear pair of angles].
From the above two equations,
∠4 + ∠5 = 180°
Similarly, ∠3 + ∠6 = 180°
Hence proved, that each pair of sameside interior angles are supplementary.
Converse of Same Side Interior Angles Theorem
The converse of the sameside interior angle theorem states that if a transversal intersects two lines such that a pair of sameside interior angles are supplementary, then the two lines are parallel.
Converse of Same Side Interior Angles Theorem Proof
Considering same above figure,
Let us assume that
∠4 + ∠5 = 180° ⇒ (1)
Since ∠5 and ∠8 forms linear pair,
∠5 + ∠8 = 180° ⇒ (2)
From (1) and (2),
∠4 = ∠8
Thus, a pair of corresponding angles are equal, which can only happen if the two lines are parallel.
Hence, the converse of the same side interior angle theorem is proved.
Important Notes
The following are the important points related to the same side interior angles.
 The same side interior angles are nonadjacent and formed on the same side of the transversal.
 Two lines are parallel if and only if the same side interior angles are supplementary.
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Same Side Interior Angles Examples

Example 1: In the given figure, 145° and 40° are the same side interior angles. Check whether the lines l and m are parallel or not.
Solution: In the given figure, 145° and 40° are the same side interior angles.
But the sum is not equal to 180° (145° + 40° =185°).
Thus, 145° and 40° are NOT supplementary, their sum is not equal to 180°.
Thus, by the "Converse of Same Side Interior Angle Theorem", the given lines are NOT parallel.
Thus, lines l and m are not parallel.

Example 2: In the following figure, l  m and (4x + 4)° and (10x + 8)° are the same side interior angles. Find the value of x.
Solution: Since l  m and t is a transversal, (4x + 4)° and (10x + 8)° are the same side interior angles.
Thus, by the "same side interior angle theorem", these angles are supplementary or we can say that their sum is equal to 180°.
Thus, (4x + 4) + (10x + 8) = 180
14x + 12 = 180
14x = 180  12
14x = 168
x = 12
Thus, the value of x = 12.
FAQs on Same Side Interior Angles
Why are the Same Side Interior Angles Congruent?
The same side interior angles are NOT congruent. They are supplementary. The same side interior angles formed when two parallel lines intersected by a transversal. The same side interior angles can be congruent only when each angle is equal to a 90 degree because then the sum of the same side interior angles is equal to 180 degrees.
Are Same Side Interior Angles Adjacent?
The same side interior angles are always nonadjacent because the angles are formed on the two different lines that are parallel to each other.
What is the Sum of the Two Same Side Interior Angles on the Transversal?
When two parallel lines crossed by a transversal they formed sameside interior angles and their sum is equal to 180 degrees. As the sum of the same side interior angles is 180 degrees therefore the angles are supplementary.
What is the Converse of Same Side Interior Angles?
The converse of the sameside interior angle states that when two lines intersected by a transversal and the angles inside on the same side are supplementary or we can say the sum of inside angles on the same side is 180 degrees then the lines are said to be parallel.
What is Another Name of the Same Side Interior Angles?
The same side interior angles are also known as consecutive interior angles as the angles are on one side of the transversal but inside the two parallel lines.
What is the Difference Between Same Side Interior Angles and Same Side Exterior Angles?
When two parallel lines are intersected by a transversal line 8 angles were formed. The same side interior angles are the angles inside the parallel lines on the same side of the transversal and the same side exterior angles are the angles outside the parallel lines on the same side of the transversal.
What is the Difference Between the Same Side Interior Angles and Corresponding Angles?
The difference between the same side interior angles and corresponding angles is corresponding angles are congruent whereas, in the case of the same side of interior angles, the sum of the same side interior angles is equal to 180 degrees only if the transversal line intersects two parallel lines.
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