If you rotate this function about the y-axis, each cross-section parallel to the x-axis will be a circle. The circumference of each circle is , and if you add up all these circumferences over then you will get the total surface area. So
If you rotate this function about the y-axis, each cross-section parallel to the x-axis will be a circle. The circumference of each circle is , and if you add up all these circumferences over then you will get the total surface area. So