jesli ktos chce zrozumiec poszerzenie
http://nptel.iitm.ac.in/courses/Webcourse-contents/IIT%20Bombay/Transportation%20Engg%201/15-Ltexhtml/p6/p.html
http://nptel.iitm.ac.in/courses/Webcourse-contents/IIT%20Bombay/Transportation%20Engg%201/15-Ltexhtml/p6/p.html