Jackson 3.7 Three point charges (q,−2q,q) are located in ....

[Pual Black]

Homework Statement

Hi,
The question is
"
Three point charges (q,−2q,q) are located in a straight line with separation a and with the middle charge (−2q) at the origin of a grounded conducting spherical shell of radius b ..."

I have found a solution for this problem but there is one step that I don't understand. I have uploaded the pdf file.
Last page the term $-\frac{1}{r}$ vanishes and at the bottom of the page the
$\frac{1}{b}-\frac{1}{r}$ vanishes too. Can you tell me why?

 

[Pual Black]

at l=0 the term vanishes.
But what about the other values of l