Are the lines parallel? co-interior angles 90 and 90

Step by step

The question

Question: find out if the two lines are parallel.

The co-interior angles are 90° and 90°.

1 Name the angle pair

A line crosses two other lines.

It is called a transversal.

Our two angles are between the lines, on the same side of the transversal: co-interior angles.

Together they make a C shape.

2 Use the parallel test

Parallel lines make co-interior angles add up to 180°.

And if they add up to 180°, the lines are parallel.

So we add them: 90 + 90 = 180.

180 is 180.

They add up to 180°.

Careful!

For co-interior angles we do not ask: are they equal?

We ask: do they add up to 180°?

So the lines are parallel.

We can draw the arrows on the lines now.

Let's look back.

First: name the angle pair.

Last: use the parallel test.