Replace the length of the corresponding line segment on the curve with the length in dx on the tangent passing through point (x, y). (Line Difference)

Or replace the length of the curve with the length of the hypotenuse of a right triangle with dx and dy as right angles:

ds=√((dx)？ +(dy)？ )=( 1+(dy/dx)？ )dx=√( 1+y')dx

Differential of surface area (edge) = rotating perimeter × above ds=2πy.ds=2πy√( 1+y')dx (rotating around X axis)

Integer =∫(x 1, x2)2πy√( 1+y')dx