bcmath

If you are on a Unix-based system of some sort there are quite a few useful command-line utilities available to you. One of these is bc, a command-line calculator. This script runs the currently selected text through bc. This allows you to type "1 + 1" in your textadept buffer, select it, run this function, and the selected text will be replaced with "2".

Place this in your ~/.textadept/init.lua

function replaceMath()
    local text = buffer:get_sel_text()
    local p = io.popen('echo "'..text..'" | bc 2>&1')
    local out = p:read('*all')
    p:close()
    buffer:replace_sel(out:gsub("\n", ""))
end

keys['ctrl+M'] = replaceMath

bc is quite powerful. Selecting the text

x = 2
2 * x

and running this function will replace the text with "4", as you'd expect.

On Debian/Ubuntu based systems you can install bc with sudo apt-get install bc.