Fundamental theorem of calculus proof
Then from the definition of A(x):

This represents the area of the region under
the graph of f and over the horizontal axis, bounded on the
left by the vertical line t = a and on the right by the
vertical line t = x + h, as shown in the diagram below.

Area under graph represents A(x + h)
Likewise,

represents the area of a similar region, but
bounded on the right by the vertical line t = x as shown in the
next diagram.
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