1. anonymous

2. anonymous

what are r and sigma representing.

3. anonymous

r is probably the vector from the origin to any point on the surface, and sigma is probably the unit vector normal to the surface at that point.

4. anonymous

@CliffSedge can you help me please

5. anonymous

Been too long since I've done integrals like these. I'll have to refresh my memory first.

6. anonymous

|dw:1350687107096:dw|

7. anonymous

|dw:1350687145672:dw|

8. anonymous

So when it says that $\vec{r}\cdot d\vec{\sigma}=0$ for three of the surfaces, can you see which three this pertains to and why?

9. anonymous

why?

10. anonymous

when, in general, is a dot product zero?