Definition of Function and Integration of a function

To understand the function, First mark the check box of Function then just move the point A and see the changes happen for the point C and B. To understand the integration of the function f(x) from l to u, First mark the check box of Integration then mark the check box of Riemann Upper Sum, Riemann Lower Sum, Riemann Integral etc... randomly. Also you can change the value of lower limit, upper limit and number of partitions by using the sliders.
Do you under the function? What the image of A? What is the point B? Riemann Lower Sum > Riemann Integration > Riemann Upper Sum, is it true? justyfy