The area function A(x) sweeps out the region under f from a to x. Shrink the strip width Δx to zero and the strip's area, divided by Δx, settles on the curve's own height — f(x). That's the Fundamental Theorem of Calculus, watched live.
Strip, magnified
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Area (pink) ÷ Δx = height (green line) — watch it meet f(x)
Controls
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A(x) fill
Rectangle
▸ Shrink Δx → 0 (auto)
Drag the orange point on the curve to move x. Drag its right edge to resize the strip directly on the canvas.