display LaTeX formula in Shiny withMathJax not working ok, how to use local MathJax in Shiny?

I saw this post R/shiny + mathjax, not rendering math

I have included

withMathJax("$a+b$"),

in my ui.R file to test LaTeX output but I'm obtaining just "$a+b$" instead of the rendered formula

I also tried

withMathJax(includeMarkdown("equations.md")),

inside equations.md I wrote "a+b" and renders ok, the problem of about that is that inside an md file I cannot link, for example this in server.R:

output$formula <- renderPrint({
      cat(paste0("$$Kc = \\frac{(a^{",1,"})^{",2,"}}{b^{",3,"}}$$"))
    })

so that I can write withMathJax(textOutput("formula")), in ui.R.

How can I instruct Shiny to use a local copy of MathJax?