Here is the question, f(x) = ce^-x , x = 1, 2, 3..... Find the value of c. Find the moment generating function of X. Use the result obtained, find E(x). Find the probability generating function of X. Verify that E(x) obtained using probability generating function is same as the first E(x) founded. I check the answer of the book, but it's wrong. Can someone help me? I'm looking for the working.