Resistance of a fructum-shaped bar

[Icheb]

I have to calculate the resistance of a bar that is shaped like a fructum and I'm only given the radius of both ends, not its length.

The formula to calculate the resistance in general is

$$R = \rho * \frac{l}{A}$$

And the formula for the volume of a fructum is

$$V = \frac{\pi * l}{3} (r^2_2 * r_2r_1 * r^2_1)$$

My problem is that I have difficulties incorporating the formula for the volume into the formula for the resistance.

Would it be possible to calculate the volume of the fructum (which would be dependent on l) and treat it like it's a cube with the same volume? That way, I could use $$A = \frac{V}{l}$$, then divide the volume by l and enter it into the formula for R.

Other than that, I have no idea what approach to take.

 

[DaveC426913]

Sorry, looked as best I can.

No idea what a fructum is.

 

[Icheb]

Sorry, I misspelled it. I meant a frustum. It's like a cone, but with the "sharp" end cut off.