The curve y=f(x) called tractrix has the following property: the derivative at any point x satisfies the formula:
f'(x) = -f(x)/
sqrt(1-(f(x))^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-sub].