Calc provides a very simple and lightweight command line calculator.
Usage: /calc <term>
e.g. /calc 1-(1-0.3)^5
Calc supports +, -, *, / and ^ (exponent), considers operator preceedence rules and handles brackets. If you need do to extract a root, remember that 9^0.5 is the same as sqrt(9).
This Addon requires LibSlash.