To introduce integrals GeoGebra offers the possibility to visualize lower and upper sums of a function as rectangles.
f(x) = x^2/4 + 2
a = 0
b = 2
n = 5
L = LowerSum[f, a, b, n]
U = UpperSum[f, a, b, n]
By chaning a, b or n (animation, 4.1.2; slider, 3.2.10) you can see the influence of these parameters. For the increment of n you should select 1 (right click on n, properties).
The definite integral can be shown as follows:
Integral[f, a, b]
The antiderivative F is created using:
F = Integral[f]