Independent trials each of which is a success with probability p are successively performed.Let Y denote the number of trials needed to obtain r success.Y is called a negative binomial random variable.Compute a)P(Y=k),k=r,r+1,... b)show that E(Y)=r/p Solution http://www.math.ntu.edu.tw/~hchen/teaching/StatInference/notes/lecture16.pdf The calculation of mean is done in the second page!.