Check the picture below, that enclosed area is pretty much our "washer".
now, to get the outer radius, or "farther" radius from the axis of rotation, what I usually do is, use the f(x) - g(x) to get the area under the curve, using the axis of rotation for g(x).
<span>The best answer is: If it is a rectangle, then it does not have three sides. </span>This statement is also known as the equivalent contra-positive.