Hi all,

I have created the left side of the picture:

with this code:

```
using Plots, LaTeXStrings, Plots.PlotMeasures
gr()
f(x) = sqrt(x)
plot(f,0,4, xtick=0:4:4, xlims=(0,6), ylims=(0,3),
framestyle=:zerolines, bottom_margin=5mm,
label=L"y = \sqrt{x}", legend=:topright)
plot!(f,0,5, label="", fill=(0, 0.15, :blue))
plot!(f,2,2.3, label="", fill=(0, 0.35, :blue))
# Annotations
annotate!([(2.12,1.6, (L"△ x", 8, :blue))])
annotate!([(2,-0.1, (L"x_{i}", 8, :blue))])
annotate!([(2.4,-0.1, (L"x_{i+1}", 8, :blue))])
```

Now I want to know how to create the right side of the picture? Because it seems to be complex, need to erase the vertical line of the \sqrt{x} at x=4 and replace with ellipse, and another ellipse in the middle with fill.