Geometry: Lines & Angles
Parallel lines, angles and triangles
Parallel Lines and Angles
When a transversal crosses parallel lines, it creates angle pairs with special properties:
• Corresponding angles are equal (F-shape)
• Alternate angles are equal (Z-shape)
• Co-interior angles are supplementary (add to 180°)
Example
Finding Unknown Angles
If two parallel lines are cut by a transversal and one angle = 65°:
• Corresponding angle = 65°
• Alternate angle = 65°
• Co-interior angle = 180° − 65° = 115°
Triangles
Angles in a triangle add up to 180°. Types: equilateral (all 60°), isosceles (two equal angles), scalene (all different). An exterior angle of a triangle equals the sum of the two non-adjacent interior angles.
Note
Remember
Angles on a straight line = 180°. Angles around a point = 360°. Vertically opposite angles are equal. Always state the reason when giving angle values in geometry.
Key Vocabulary
Parallel linesLines that never meet, always the same distance apart
TransversalA line that crosses two or more parallel lines
Corresponding anglesAngles in the same position at each intersection (equal if lines are parallel)
Alternate anglesAngles on opposite sides of the transversal between parallel lines (equal)
SupplementaryTwo angles that add up to 180°
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Parallel lines
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