Question

What are co-interior angles?

Answer 1

Hint: Here, in the given question, we are asked to define and tell about the interior angles. Before answering this question, we must understand the terms used i.e. co-interior and angles. Let us know about these two terms first.
Angle: An Angle can be defined as a figure formed by two lines or rays having a common endpoint.
Co-interior: As the word suggests, interior means inside and “co” here stands for consecutive meaning on the same side.
Now, let us understand in detail.

Complete step-by-step solution:
Before defining the co-interior angles let us first understand the term transversal.
Transversal: A line that intersects two or more other lines is called transversal.

Co-interior angles can be defined as a pair of angles between the two lines and on the same side of the transversal. Let us better understand co-interior angles through a diagram.
 

In the diagram, the pair of angles which are marked are co-interior angles.

Note: There is a unique property for the co-interior angles. If there are two parallel lines and one line intersecting them called transversal, then the sum of interior angles will be \[180^\circ \] or are supplementary. Like, in the diagram given below the sum of \[\angle a\] and \[\angle b\] will be \[180^\circ \]. Or we can say that \[\angle a\] and \[\angle b\] are complementary.

But, it is important to remember that this property will be applicable only and only when transversal is cutting two parallel lines.
Also, the converse of the property holds true. It means if the co-interior angles are complementary, then the two lines must be parallel.