Answer:
Co-interior angles or Consecutive interior angles are the two angles that are on the same side of the transversal. Co-interior angles are the interior angles and it sums up to 180 degrees. It means that the sum of two interior angles, which are on the same side of transversal is supplementary. Co-interior angles resemble like in “C” shape and both the angles are not equal to each other. The co-interior angle is also known as the consecutive interior angles or the same side interior angles.Co-interior Angles
Co-interior Angle Theorem and Proof
Statement:
If the transversal intersects the two parallel lines, each pair of co-interior angles sums up to 180 degrees (supplementary angles).
Proof:
Co-interior Angles Proof
Let us consider the image given above:
In the figure, angles 3 and 5 are the co interior angles and angles 4 and 6 are the co-interior angles.
To prove: ∠3 and ∠5 are supplementary and ∠4 and ∠6 are supplementary.
Given that, a and b are parallel to each other and t is the transversal.
By the definition of linear pair,
∠1 and ∠3 form the linear pair.
Similarly, ∠2 and ∠4 form the linear pair.
By using the supplement postulate,
∠1 and ∠3 are supplementary
(i.e.) ∠1 + ∠3 = 180
Similarly,
∠2 and ∠4 are supplementary
(i.e.) ∠2 + ∠4 = 180
By using the corresponding angles theorem, we can write
∠1 ≅∠5 and ∠2 ≅ ∠6
Thus, by using the substitution property, we can say,
∠3 and ∠5 are supplementary and ∠4 and ∠6 are supplementary.
Hence, the co-interior angle theorem (consecutive interior angle) is proved.
The converse of this theorem is “if a transversal intersects two lines, such that the pair of co-interior angles are supplementary, then the two lines are parallel”.