Well, this is a well known sum: .
Does that help you understand?
This is a question from Degroot and Schervish's Probability and Statistics Textbook: p 103 qn 9
Suppose that a random variable X has a discrete distribution with the following probability function:
f(x) = ( c/(x squared) for x=1,2,...
( 0 otherwise
Find the value of the constant c.
I tried working it out and my answer is c = 1.
But the back of the textbook states the answer as 6 / pi squared.
Please could someone help me understand why?