The reason for dxdy = rdrdθ is that dx dy is a small region of the rectangular plane, so we need to use the corresponding small region of the polar coordinate system. But the problem is that the area of the regions with length difference dr and angle difference dθ is not a constant, it's like the outer part of a sector, so we need to use the equation of sector area to figure out the area. The larger one has angle dθ and length r+dr, so its area is ((r+dr)^2 * dθ)/2, while the smaller one has angle dθ and length r, so its area is (r^2* dθ)/2, the difference will be (2rdr + dr^2)* dθ/2, and when we extract dr*dθ, the remaining one will be (2r+dr)/2. Since dr approaches 0 when the slices get smaller and smaller, (2r+dr)/2 will approach 2r/2 = r, so the final result is dxdy = rdrdθ
This dude has a natural ability to explain things the easy, understandable way. I watched 2 other videos of this same function prior to this one, but too many gaps in the explanation.
this integral is why i love calculus. you can really feel each step of the process and see why integration is so powerful. honoring symmetry, translating into polar coordinates, landing on a solvable form that provides such a satisfying answer. thanks prime newtons! never stop learning :)