The surface of the triangle is $X$ and the circumfrence of the base of the cylinder is $Y$.
What is the volume of the cylinder ?
The answer is $XY$ but why ?
If you'll take the triangle and spin it all the way around the circumference of the cylinder, you'll get the volume of the inner cone. Furthermore, let's build an extra triangle like so:
Multiply that rectangle by the circumference, we'll get that the volume of the cylinder is: $2XY$ which is not the answer.
This is what we have: $$ X = \text{Area of triangle} = rh/2\\ Y = \text{Circumference} = 2\pi r\\ XY = \pi r^2 h $$ But we also have $$ \text{Volume} = \pi r^2h $$ so we see that they are equal.