How can I simplify repeated pow() function? :  

How can I simplify this?

    function x(a, b, c){
        rv = Math.floor(a * Math.pow(2, b));
        for (i = 1; i < c; i++){
            rv += Math.floor(a* Math.pow(1.2, b+i));
        }
        return rv;
    }
I'm using da PC and on da WINDOWS 64 bit thingy.. please hel me i am bery stuck here..