<aside> 💡 top - bottom or right - left for it yields out the distance between the 2 curves.

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Exact Area

In this case $f(a+\Delta x i)$, representing the height of the graph (the y output), is replaced by f(x)-g(x)

Horizontal slice

By minus the right with the left, the distance between the functions can be yield out always positively.

Vertical slice

By minus the top with the bottom, the distance between the functions can be yield out always positively.