3-(1)

$\displaystyle \mbox{div} \mathbf{a}$	$\textstyle =$	$\displaystyle \frac{\partial (xy)}{\partial x} + \frac{\partial (yz)}{\partial y} + \frac{\partial (zx)}{\partial z }$	(33)
	$\textstyle =$	$\displaystyle y + z + x$	(34)
	$\textstyle =$	$\displaystyle x + y + z$	(35)