When we were in school they told us that the Surface Area of a sphere = $4\pi r^2$
Now, when I try to derive it using only high school level mathematics, I am unable to do so. Please help.
Geometry – Can Surface Area of a Sphere Be Found Without Integration?
3dgeometry
Best Answer
Imagine a vertical cylinder enclosing the sphere, with height $2r$, radius $r$, and open ends. This cylinder has surface area $4\pi r^2$. The trick is to show that if you slice the cylinder and the sphere into infinitesimally thin horizontal rings, then at a given height, the surface area of the spherical ring equals the surface area of the cylindrical ring. Thus the total surface areas are equal.
Suppose the cylindrical ring has has height $\delta h$, and therefore area $2\pi r \times \delta h$. If the ring is at a height $r\sin\theta$ above the equator of the sphere, with $-\pi < \theta < \pi$, then the spherical ring has radius $r\cos\theta$, but its surface is at an angle $\theta$ from the vertical. So its area is $2\pi r \cos \theta \times \delta h/\cos \theta$, which is the same as the cylindrical ring.
This really needs some nice pictures, but I have no skill in that direction.