Am using the following jquery in my application,

and also i have added 

Jquery-1.5.1.js

Jquery-1.5.1.min.js

so application working properly, but i felt don't need two versions of the js in single application, so i have removed the jquery-1.5.1.js and Jquery-1.5.1.min.js.

but after removed the application giving the exception, exceptions are

1. uncaught reference error: jquery is not defined

2. uncaught reference error: $ is not defined

then i have added the jquery-1.5.1.js and jquery-1.5.1.min.js, now the application working fine.

Here, am unable to find the cause, can u please give me the cause?

I have used the latest js file in application (jquery-ui-1.8.11.min.js), 

even should I use the older version? 

should it need(Jquery-1.5.1.js and Jquery-1.5.1.min.js)?