Vector Algebra

$$\displaystyle \left ( x+y \right )\left ( dx-dy \right )= dx+dy.$$
$$\displaystyle \left ( x+y \right )\left ( dx-dy \right )= dx+dy.$$
$$\displaystyle \left ( x+y \right )\left ( dx-dy \right )= dx+dy.$$
$$\displaystyle \left ( x+y \right )\left ( dx-dy \right )= dx+dy.$$