Step by step
The question
Question: find out if the two lines are parallel.
The co-interior angles are 2x and 3x, with x = 30.
1 Put x in
Put x = 30 in the angle 2x.
That is 60°.
Put x = 30 in the angle 3x.
That is 90°.
2 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.
3 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: 60 + 90 = 150.
150 is not 180.
They do not 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 not parallel.
If we carry the lines on, they would meet.
Look at the picture: the top line is turned.
Let's look back.
First: put x in.
Then: name the angle pair.
Last: use the parallel test.