Question

Wait, I thought rectangles hadan area of l x w, what's goingon here? Why do you thinkwe're using 2nr instead of l?

Answer 1

The tube part of a cylinder looks like a rectangle in the flat net. To make the cylinder, it has to be curved around to follow the edges of the circulars ends:

So, we can find the surface area of a cylinder as

$SA=2\pi\cdot r^2+(2\pi r)(h)$