Introduction to Lines & Angles (Cambridge International IGCSE Mathematics)

Properties of Lines & Angles

What properties do I need to know about lines?

• A line is a straight, one dimensional path that extends forever

  • A line segment is a part of a line, it has a start and an end point

  • A line segment that starts at the point A and ends at the point B is usually labelled AB

  • Two lines with single marks are of equal length

    • Two other lines with double marks are of (a different) equal length

• Two lines are parallel if they continue in the same direction as each other forever

  • Parallel lines never intersect(meet)

  • Parallel lines should be marked with arrows

    • If there is more than one pair (or set) of parallel lines in a diagram then multiple arrows will be used

• Two lines are perpendicular if they intersect at right-angles (90°)

What properties do I need to know about angles?

• When two lines intersect(meet) they will form an angle

• There are different types of angles

  • An angle that is less than 90° is called an acute angle
  • An angle that is exactly 90° is called a right angle
  • An angle that is greater than 90° and less than 180° is called an obtuse angle
  • An angle that is exactly 180° is called a straight angle
  • An angle that is greater than 180° but less than 360° is called a reflex angle

• Angles at a point add up to 360°

• Angles on a straight line add up to 180°

  • Angles that add up to 180° are called supplementary angles

    • e.g. 30 and 150 degrees are supplementary angles

Exam Hint

• Do not assume two lines are parallel or perpendicular just because they look it, always look out for the arrows or right angle in a diagram, or read the question to look for clues

Worked Example Write down the mathematical name for an angle which is greater than 90° but less than 180°.

Answer:

Final answer: An obtuse angle