a circle is actually a rectangle in disguise
(maybe played without sound and stop at 1:25)
several useful Geogebra applets, show how the area formula can be obtained from an infinite number of tiny sectors
this one by Anthony Or uses few words
this shows how the area of an internal regular polygon approaches the area of the circle as the number of sides increases (Archimedes)
a circle is actually an isosceles triangle in disguise
what will the base of the triangle be?
and the height?