Halving instead of doubling for angle at centre H
"The Half-Double Confusion"
The Mistake in Action
Angle ACB (at circumference) = 50°. Find angle AOB (at centre).
Wrong: Angle AOB = 50° ÷ 2 = 25°
Why It Happens
Students remember there's a factor of 2 involved but apply it the wrong way round. They halve when they should double.
The Fix
Angle at centre = 2 × angle at circumference
The angle at the centre is the bigger one. Think of it as the centre having the "full view" of the arc.
Angle ACB = 50° (at circumference) Angle AOB = 50° × 2 = 100° (at centre)
Memory aid: "Centre sees more" — double it!
Spot the Mistake
Can you identify where this student went wrong?
Angle ACB (circumference) = 50°
Angle AOB = 50° ÷ 2 = 25°
Click on the line that contains the error.
Related Topics
Learn more about the underlying maths: