Simplify each of the expressions in parts (a) through (c) below.

4. What expression would go in the box in order to make the equation
   $\square ( \frac { 5 } { x } + \frac { 8 } { y } ) = 5 y + 8 x$ true?

   Hint (d):

   $\frac{5xy}{x}+\frac{8xy}{y}=5y +8x$