private String getSelectedExpression()
    {
        String result="";
        switch(Equation.getSelectedIndex()+1)
        {
            case 1:
                result=fx;
                break;
            case 2:
                result=gx;
                break;
            case 3:
                result=ax;
                break;
            case 4:
                result=bx;
                break;
            case 5:
                result=cx;
                break;
            case 6:
                result=dx;
                break;
            case 7:
                result=ex;
                break;
        }
        return result;
    }