Could you explain a little bit more where you got everything from? For instance the 2y? Also how did you get (2 / 5) = (r / y) Also from your final line, where is that final y coming from?
*sigh* this is really tough for me to understand so hopefully i'm not annoying you .
Additional question is. Why are we using the area of a circle when integrating down for the discs and using the volume of a cone as we go up? Can we not use the area of a circle as we're going up since it seems like the same thing? If not what's the reason? Also why did the professor multiply the distance from the disc and up (y) to everything?