Mixing up alternate and co-interior angles
"The Z and C Confusion"
The Mistake in Action
Lines AB and CD are parallel. One angle is 70°.
Wrong: $x = 70°$ (treating co-interior as alternate)
Why It Happens
Students recognise a parallel lines setup but apply the wrong rule. Alternate angles (Z-shape) are equal, but co-interior angles (C/U-shape) add to 180°.
The Fix
Alternate angles (Z-shape): angles on opposite sides of the transversal → EQUAL
Co-interior angles (C-shape): angles on the same side of the transversal, between parallel lines → ADD TO 180°
If the angles are co-interior: $$x + 70° = 180°$$ $$x = 110°$$
Tip: Trace the shape with your finger. Z-shape = equal. C-shape = sum to 180°.
Spot the Mistake
Can you identify where this student went wrong?
Lines AB and CD are parallel. Angle ABC = 70°. Find angle BCD (co-interior).
ABC and BCD are co-interior angles
So angle BCD = 70°
Click on the line that contains the error.
Related Topics
Learn more about the underlying maths: