Have you tried reading these threads:

http://www.mathhelpforum.com/math-he...tribution.html

http://www.mathhelpforum.com/math-he...-function.html

http://www.mathhelpforum.com/math-he...-interval.html

http://www.mathhelpforum.com/math-he...-interval.html

If this doesn't help, I'll try to find the time later to say something specific about your question.