Good visualization website: https://www.geogebra.org/m/Fayr3trM
Disc Method: Given a function, we rotate this around the x-axis (or y-axis), creating multiple discs/circles from the start to the end of the functions. Summing up the area of these circles (recall: πr²).
Volume of the Solid of Revolution, the Disc Method (cr: Blackpen Redpen)
Washer Method: This is similar to the disc method, HOWEVER, the rotation will create 2 circles. A inner (hollow portion) and outer circle. We are looking for the “donut” solid therefore it is ∫(area of outer circle -area of inner circle)dx.
Generalizing the washer method (cr: Khan Academy)