0580
Geometry Toolkit
Geometry
Describing lines
- A line segment is the part of a line between two named points, so the segment from A to B is written AB
- Two lines are parallel when they run in the same direction and never meet, and arrows on a diagram mark them as such
- Two lines are perpendicular when they meet at a right angle, shown by a small square in the corner
- Equal lengths are marked with matching dashes: one dash for one pair, two dashes for a different pair
- Never judge a diagram by eye, since most are printed "NOT TO SCALE" and the markings carry the information
- A perpendicular bisector cuts a segment in half at a right angle, and a bisector of an angle cuts it into two equal parts
Naming angles by size
| Name | Size |
|---|---|
| Acute | less than 90° |
| Right angle | exactly 90° |
| Obtuse | between 90° and 180° |
| Straight angle | exactly 180° |
| Reflex | between 180° and 360° |
- Angles on a straight line add to 180°, and angles meeting at a point add to 360°
- Two angles adding to 180° are supplementary, and two adding to 90° are complementary
- Interior angles are inside a shape and exterior angles lie between one side and the extension of the next
- Three-letter notation names an angle by the point at its vertex in the middle, so angle ABC has its vertex at B
