Electric Potential for an Infinite line charge question

[Impoor]

Homework Statement

This may be simple but apparently I can't see it. When given two parallel infinite line charges, how do you change dl to dr? A book I have says that dl*\hat{r}=dr but I don't see how. Any help is greatly appreciated. Thanks!

Homework Equations

The Attempt at a Solution

Homework Statement

Homework Equations

The Attempt at a Solution

 

[olgranpappy]

it would be easier to answer you if you explain in more detail what you mean by "dl" and "dr" and "*".

 

[rohanprabhu]

as olgranpappy said.. it is necessary that you provide us with the EXACT question since the title implies a single infinite line charge.. however you mention 2 infinite line charges.