Step by step
The question
Question: find out if the two lines are parallel.
The co-interior angles are 100° and 75°.
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: 100 + 75 = 175.
100 + 75 = 175
175 is not 180.
They do not add up to 180°.
175 ≠ 180
Careful!
For co-interior angles we do not ask: are they equal?
We ask: do they add up to 180°?
Careful: co-interior: add to 180°, not equal
So the lines are not parallel.
Answer: = 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: name the angle pair.
Last: use the parallel test.
Answer
Answer: = not parallel