A Problem of Roofing Paper

           For doing the interior decorations of my apartment, I have bought a roll of roofing paper. I do know its exact thickness, but let us assume it is X. How can I find out how long the roll is without unrolling? In other words can, you derive a formula for length of roll if the radius is r?

         The area of the edge of the paper which is a rectangle equals the area of the end of the roll. If the radius of the roll is R, then R² - w 1 or 1 = R² / W

Answer:   The area of the edge of the paper which is a rectangle equals the area o the end of the roll. If the radius of the roll is R, then R? = W1 or 1 = R² /w