Angles in parallel linesCo-interior angles
Name: Date:
The two lines are parallel. Find x.
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Co-interior angles worksheets
Co-interior angles sit between two parallel lines, on the same side of the line crossing them, and unlike the other two pairs they are not equal: they add to 180°. These free printable worksheets draw the figure, mark one angle, and ask for its partner.
If they get stuck
Co-interior angles
The answer key prints on a separate page.
What's on this sheet
American textbooks usually call them same-side interior or consecutive interior angles; co-interior is the name used in the UK and Australia. The picture is the letter C, sometimes facing backwards, with one angle tucked into each of its corners. Finding the C matters more here than finding the F or the Z, because copying the marked number — the right answer for corresponding and alternate angles — is the wrong answer here every time.
Why this sheet works
Why the pair adds to 180° takes one step from a fact a child already has. The co-interior angle at the top crossing equals its alternate partner at the bottom crossing, and that partner sits beside the other co-interior angle on a straight line. Angles on a straight line make 180°, so the co-interior pair does too.
How this one works
55° and x
55°andx
co interiorangles
180 − 55=125
x=125°
- Start with the arrows: they say the two lines are parallel. Without that, none of the angle facts apply and this picture would not have an answer.
- Now find where the two marked angles sit. They are between the two parallel lines, on the SAME side of the slanted one. That makes them co interior angles.
- Co-interior angles add up to 180° — this is the one pair that is NOT equal, so the answer is not the number you were given. 180 − 55 = 125, so x = 125°.