Help  Why is it 1/3 times the area of the circle here?

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[Originally posted in the Discussions]
Why is it 1/3 times the area of the circle here? Where does the circle come in because the shaded area doesn't even cover the whole circle? And why just multiply it by 1/3?

Prof. Loh is referring here to the area of the green wedge, which is shaped like a big pizza slice. The "V" angle of the slice is of measure 120 degrees, which is onethird of the circle (a circle has 360 degrees in total).
Imagine you were cutting a huge apple pie into three gigantic slices; each person would get onethird of the whole piece, and each person's piece would be approximately the same shape as the green wedge above.
Happy Learning,
The Daily Challenge Team