This is Calculus.

#calculus #circle

[ogreeni]

The slice is not a rectangle. It is more of a trapezoid.

[eggxecution]

this is a better visualization than others

[nictactoe]

There's actually a much more simpler proof. You can take the radius of the circle and make numerous sectors out of the circle and arrange the sectors into a rectangular shape. If you were to have an infinite number of really small sectors laid out this way, you'll find that a rectangle is formed of length πr and width r - the area of that rectangle is the same of a circle.