Area Under a Curve
Drag the slider to change the x-coordinate of point A. Watch the area beneath the curve grow. Notice that the area under the curve is equal to the y-coordinate of point A. Turn on g(x) on the left to see the intergal of f(x).
You can try other functions for f(x) by double-clicking f(x) on the left. I suggest reloading the page before doing that.