This a method mainly for finding

volumes of

irregular solids.

First off, we define our coordinate system.
The positive X-axis is coming out of your screen towards you,
the positive Y-axis heading off to your right,
and the Z-axis is pointing straight up.
Let's suppose we have some kind of solid body, say a peanut of height *h*.
We define A(x) to be it's cross-sectional area at a height
*x* (Assuming the object is sitting on the XY-plane).
The very simply, the volume of the afore mentioned peanut is:
/*h*
volume= | A(x) *d*x
/0
whose approximating Reimann sum is:
__n-1__
\
__/____ A(c{*i*})(x{*i*+1}-x*i*)
*i*=0
Note: Braces indicate a subscript