text: Q:
  
  Find the volume of the triangle with vertices $(1,1)$, $(1,2)$, and $(2,2)$ about the y-axis using the disks method.
  
  Note that we can find volume of this by the disk method: $\int _{a}^{b}\pi f\left( x\right) ^{2}dx$ but I could't a function $f(x)$. Can you give a hint?
  
  A:
  
  $$V=\text{Outside}-\text{Inside}=\int_1^2\pi f(x)^2dx-\int_1^2\pi\cdot1^2dx$$
   $$=\int_1^2\pi(x^2-1)dx$$
   $$=\pi\bigg[\dfrac{x^3}3-x\bigg]_1^2=\dfrac{4\pi}3$$
  
  