This study notes covers Worked Example 3: Co-interior Angles within Angles in Parallel Lines for GCSE Mathematics. Revise Angles in Parallel Lines in Geometry & Measures for GCSE Mathematics with 12 exam-style questions and 3 flashcards. Use this page as part of a wider topic revision path rather than treating it as an isolated fact. It is section 6 of 9 in this topic. Use this study notes to connect the idea to the wider topic before moving on to questions and flashcards.
Worked Example 3: Co-interior Angles
Angle a = 72°. Find the co-interior angle b.
Step 1 Recall the rule for co-interior angles
Co-interior angles make a C-shape
Co-interior angles add up to 180°
Step 2 Calculate b
a + b = 180°
72° + b = 180°
b = 180° − 72° = 108°
Reason: Co-interior angles on parallel lines
Practice questions for Angles in Parallel Lines
Two parallel lines are cut by a transversal. Which statement about alternate angles is correct?
State the THREE angle rules that apply when a transversal crosses two parallel lines. For each rule, state whether the angles are equal or supplementary.