region R is bounded by the curve y=ln x, and the lines y=1 and x=1. Find the volume of the solid formed when R is rotated through 360 degree about the y-axis.
What is the graph look like and how to integrate ?
Use the washer method for this problem, or the shell method if you are familiar with both. Below I quickly discuss the shell method:
V = INTEGRAL ( distance traveled * height of region)
distance = 2 pi x since the radius from the rotational axis to a level x is simply the x coordinate
height of region = 1 - ln x
the bounds are from x = 1 to the intersection point between y=ln x and y=1, which is simply e = 2.71828...
So the volume is INTEGRAL (2pi x (1-ln x)) from 1 to e.
The washer method, briefly:
Inner radius: r(y) = 1
Outer radius R(y) = x = e^y
Bounds y = 0 to y =1
Formula: V = INTEGRAL (Pi R^2 - Pi r^2)