Circle theorems
Rules about angles inside circles. One example: an angle drawn from the two ends of a diameter to any point on the circle is always 90 degrees.
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Analogy
Like a rope pulled tight across a circular room: the angle always stays a right angle.
Example
Triangle on a diameterOne angle is 90Others: 35 and 5535 + 55 + 90 = 180
In practice
Craftspeople use this to find the exact centre of a round object using a set square.
Practice
Flashcards
- What is the angle in a semicircle?90 degrees.
Questions people ask
What is the angle in a semicircle?
It is always 90 degrees. If you join both ends of a diameter to any other point on the circle, the angle at that point is a right angle. This is one of the most-used circle theorems and it is simple to apply.
What does the angle at the centre theorem say?
The angle at the centre of a circle is twice the angle at the edge when both are drawn from the same two points. If the angle at the edge is 40 degrees, the angle at the centre is 80 degrees.