In this New York Times article, Steven Strogatz offers the following argument for why the area of a circle is $\pi r^2$. Suppose you divide the circle into an even number of pizza slices of equal arc length, and wedge them together in such a way that half of the slices have an arc at the bottom, and half of the slices have an arc at the top:
Then, the base of the shape created has length $\pi r$, and its height is $r$. As the number of slices tends to infinity, the limiting case is that of a rectangle:
Hence, the area of the circle is $\pi r^2$. Although this argument is very geometrically appealing, it also seems fairly difficult to make rigorous. I suppose the most challenging part is showing that the base of the shape really does become arbitrarily flat, and its height becomes arbitrarily vertical, if that makes sense. How might we convert this intuitive argument into a rigorous proof?
Best Answer
For simplicity I will assume the number of slices $n$ is even.
If you connect the four "corners" of the wedge figure, you obtain an "inner" parallelogram with one pair of sides having length exactly $r$, and another pair of sides having length approximately $\pi r$. The height of the parallelogram is $r \cos \frac{\pi}{n}$ (which tends to $r$); the length is $n r \sin \frac{\pi}{n}$ (which tends to $\pi r$). So the area of the "inner" parallelogram tends to $\pi r^2$.
You can also consider inscribing the wedge figure inside a slightly larger parallelogram. The height is again $r \cos \frac{\pi}{n}$ but with two additional "crusts" each having thickness $r(1 - \cos \frac{\pi}{n})$, so the final height is $r(2-\cos \frac{\pi}{n})$ (which also tends to $r$). I think the length is the same as the that of the inner parallelogram: $nr \sin \frac{\pi}{n}$ (which tends to $\pi r$).
So the area of the wedge figure is between the areas of the two parallelograms $\pi r^2 cos \frac{\pi}{n} \frac{\sin(\pi/n)}{\pi/n}$ and $\pi r^2 (2-\cos \frac{\pi}{n}) \frac{\sin(\pi/n)}{\pi/n}$. Since both of these areas converge to $\pi r^2$, so does the area of the circle.
Picture of the inner and outer parallelograms: