geometry · Year 6 · AC9M6M04
Vertically opposite angles
When two straight lines cross they make an X with four angles. The angles across from each other are vertically opposite, and they are equal. Angles next to each other sit on a straight line, so they add up to 180°. First decide which kind of pair you have. With more lines, carry a known angle across to its opposite, then use a straight line.
Sample questions
Two straight lines cross. Find the angle marked with a question mark. The diagram is not drawn to scale.
- The ? is vertically opposite the 55° angle: they sit across from each other where the lines cross.
- Vertically opposite angles are equal.
- So the angle is 55°.
Answer: 55
Two straight lines cross. Find the angle marked with a question mark. The diagram is not drawn to scale.
- The ? is vertically opposite the 35° angle: they sit across from each other where the lines cross.
- Vertically opposite angles are equal.
- So the angle is 35°.
Answer: 35
Two straight lines cross. Find the angle marked with a question mark. The diagram is not drawn to scale.
- The ? is vertically opposite the 55° angle: they sit across from each other where the lines cross.
- Vertically opposite angles are equal.
- So the angle is 55°.
Answer: 55