Example 1 · level 1
Lines l and m are parallel and a transversal crosses them. One angle is 80°. Find the corresponding angle to it (in degrees).
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- Corresponding angles are equal, so the angle is 80°.
Answer: 80
CLASS 9 MATHS · LINES AND ANGLES
Angles made by a transversal with parallel lines
With parallel lines: corresponding angles are equal, alternate interior angles are equal, and co-interior angles add to 180°.
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Worked examples
Example 2 · level 2
Lines l and m are parallel and a transversal crosses them. One angle is 42°. Find the co-interior (same-side interior) angle to it (in degrees).
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- Co-interior angles: 180° − 42° = 138°.
Answer: 138
Example 3 · level 3
Two parallel lines are cut by a transversal. A pair of alternate interior angles measure (5x + 15)° and (x + 103)°. Find x.
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- 5x + 15 = x + 103
- 4x = 88
- x = 22. (Each angle = 125°.)
Answer: 22