This is a Sec 3 topic, but I try to explain them as simple as possible at the Sec 2 level.

Imagine you have a pizza (full circle) and you cut them into slices. Suppose you cut them into six equal slices. Now, what happens?

The angle at the centre of the circle gets cut into six equal angles (obviously), so that the angle made by each slice 1/6 of the full 360 degrees, or equivalently 60 degrees.

Now, the area of the pizza that you have taken is also 1/6 of the total area of the full circle.

But that is not it yet. The length of the arc, which is basically the curved part of the pizza crust (from the circumference of the full pizza), is also cut into six equal lengths, so the length of the arc (crust) is also 1/6 of the circumference of the pizza.

The same ideas apply when the pizza is sliced into 8 parts, 12 parts or n parts.