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
- 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
- Angles that add up to 180° are called supplementary angles
Exam Tip
- 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°.
An obtuse angle
