Geometry of 2D Shapes – Part 1

About this video

An introduction to the angle relationships on straight lines and around a point — the reasoning rules every geometry answer depends on.

What it covers

• Angles on a straight line add to 180°
• Angles around a point add to 360°
• Vertically opposite angles are equal
• Stating reasons correctly

Exercises to try

Work each one out, then click to reveal the answer.

1. 1

   Two angles on a straight line: one is 65°. Find the other.

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   180° − 65° = 115°
2. 2

   Three angles around a point: 120°, 150° and x. Find x.

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   360° − 120° − 150° = 90°
3. 3

   Two lines cross; one angle is 40°. What is the vertically opposite angle?

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   40° (vertically opposite angles are equal)