I reckon you mean a Riemann sum?.
If we calculate the area of an infinite number of rectangles, we approach the area under the curve, such as it is.
Right endpoint method: \(\displaystyle a+k{\Delta}x\)
\(\displaystyle {\Delta}x=\frac{4-0}{n}\)
Thus, rectangle k has area: \(\displaystyle f(x_{k}){\Delta}x=5{\Delta}x=5\frac{4}{n}=\frac{20}{n}\)
The sum of the areas is \(\displaystyle \sum_{k=1}^{n}\frac{20}{n}=\frac{1}{n}\sum_{k=1}^{n}20=20\)
