Update 3-julia_documentation.md

docs/src/lecture/3-julia_documentation.md

@@ -29,6 +29,12 @@ Compute the sum of `x` and `y`.
 - `x::Int`: The first integer
 - `y::Int`: The second integer
 
+## Mathematical formula
+
+```math
+  (x,y) \\mapsto x+y
+```
+
 # Examples
 ```julia
 julia> add(2, 3)