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Angles in Parallel Lines

≈ 35 min
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Angles in Parallel Lines

When a transversal crosses parallel lines, matching angle positions have predictable relationships. Corresponding angles are equal, alternate angles are equal, and co-interior angles add to 180∘180^\circ. Use arrow marks to show the parallel lines and name the relationship before doing a calculation.

Worked example

If a corresponding angle is 72∘72^\circ, the matching corresponding angle is also 72∘72^\circ. Its adjacent angle on a straight line is 180−72=108∘180-72=108^\circ.

Exam method

  1. Mark the parallel lines. 2. Locate the two angles in relation to the transversal. 3. State equal or supplementary, then calculate.

Two corresponding angles are formed by parallel lines. One is 58∘58^\circ. Find the other.

Co-interior angles are xx and 115∘115^\circ. Find xx.

Which condition must be shown before using corresponding-angle equality?

Complete: Alternate angles in parallel lines are ______.