Question

The curve y=f(x) called Tractrix has the following property" the derivative at any point x satisfies the formula:
f'(x) = -f(x)
(1-f(x)^2)^(1/2)
Consider the region under the tractrix, within the limits 0<=x<=a. Find the volume of the solid obtained by revolving this region around the x=axis in terms of the constant c=f(a). [Hint: use u-sunstitution]