# Integration question.. please help!A part of the curve y = a/x, where a is a positive constant, is rotated through 360◦ about the x-axis between x =1 and x=3. Find the value of 'a' if the volume...

A part of the curve y = a/x, where a is a positive constant, is rotated through 360◦ about the x-axis between x =1 and x=3. Find the value of 'a' if the volume obtained is 24pi.

I don't know how to integrate the curve y = a/x.. please help me solve the question above with an explanation.
Thanks!

sciencesolve | Teacher | (Level 3) Educator Emeritus

Posted on

You need to use the equation that gives the volume of solid of revolution, such that:

`V = pi*int_a^b y^2 dx`

You should notice that the problem provides the limits of integration, `a = 1` and `b = 3` , the value of volume `V = 24 pi` and the function `y = a/x` such that:

`24pi = pi*int_1^3 (a/x)^2 dx => 24 = a^2 int_1^3 1/x^2 dx`

`24 = a^2 int_1^3 x^(-2) dx => 24 = -a^2*(1/x)|_1^3`

Using the fundamental formula of calculus yields:

`24 = -a^2*(1/3 - 1/1) => 24 = (2/3)a^2 => a^2 = (3*24)/2`

`a^2 = 36 => a = +-6`

Since the problem specifies that a is a positive constant yields that `a = 6` .

Hence, evaluating the value of positive constant a, under the given conditions, yields `a = 6.`