Why use sin^2() in calculating a potential and how can you go from 3 to 5?

[Bert]

why do the use sin^2()? en who can you go from 3 to 5.
http://img413.imageshack.us/img413/3919/verband1bismy6.jpg
http://img254.imageshack.us/img254/6993/verband2sk4.jpg

Thanks.

 

[Kurdt]

Looks to me like a simple error in the book. Should be $sin(\theta)$ rather than sin squared.

 

[Bert]

I thinks also but I'm not certain.

Thanks.