Math Help - finding the lateral surface area

1. finding the lateral surface area

the book says r(x) = x? i dont understand why, i thought r(x) would be the length of the strip, (3/4)x - 0. well take a look at the problem and tell me what you think. thx in advance.IMG_0001.pdf

2. Originally Posted by slapmaxwell1
the book says r(x) = x? i dont understand why, i thought r(x) would be the length of the strip, (3/4)x - 0. well take a look at the problem and tell me what you think. thx in advance.IMG_0001.pdf
Look at the way "r" runs. y= (3/4)x is the y-coordinate of a point on the line. Since you are rotating around the y-axis, the radius of each circle is the x-coordinate of the point.