OpenOlympiad
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Parallel lines & transversal

A transversal is a line that crosses two other lines. When those two lines are parallel, special angle pairs appear.

  • Corresponding angles (in same position at each intersection) — EQUAL.
  • Alternate interior angles (between the parallels, opposite sides of transversal) — EQUAL. Look for a "Z" shape.
  • Co-interior (same-side interior) angles — sum to 180°. Look for a "C" shape.
Example
If angle a = 70°, then:
Corresponding e = 70°.
Alt-interior c and f = 70°.
Co-interior c + f = 180° — wait that's alt. Co-interior pair: c and e → 70° + 110° = 180°. ✓
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💡 Tip:Draw a Z — its equal corners are alternate angles.
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5 questions to check what you just read.

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  1. Q1.When parallel lines are cut by a transversal, corresponding angles are:
  2. Q2.Alternate interior angles form a shape like:
  3. Q3.Co-interior (same-side interior) angles are:
  4. Q4.If one angle made by a transversal with two parallel lines is 70°, an alternate angle equals:
  5. Q5.A transversal intersects two lines and alternate interior angles are equal. The two lines are: