# Deriving cross product and dot product, stuck at beginning.

by JJRKnights
Tags: beginning, cross, deriving, product, stuck
 Emeritus Sci Advisor HW Helper Thanks PF Gold P: 11,722 No, that isn't correct. You can't treat $\phi$ like a constant.
 Emeritus Sci Advisor HW Helper Thanks PF Gold P: 11,722 Just start with the definition of the divergence and apply it to ϕF = (ϕP, ϕQ, ϕR): $$\nabla\cdot(\phi \mathbf{F}) = \frac{\partial}{\partial x} (\phi P) + \frac{\partial}{\partial y} (\phi Q) + \frac{\partial}{\partial z} (\phi R)$$Now use the product rule on each of the three terms.