The joint probability distribution of random variables X and Y is given by f{(x,y)}=k * x * y for x = 1,2,3, y = 1,2,3, and k is a positive constant. What is the probability that X+Y will exceed 5?
解析
The function x*y is described in the following table. The sum of the entries is 36. The scaling factor k must be such that the total probability is 1. Therefore, we have k=1/36. The table shows one instance where X+Y > 5，which is x=3, y=3. The probability is p = 9/36 = 1/4.
f{(x,y)}是什么意思，f{(x,y)}=k * x * y又是什么意思？The scaling factor k must be such that the total probability is 1这个又是为什么？The probability is p = 9/36 = 1/4为什么这么算？
X和Y分别123取值，那不就是1/3*1/3=1/9么？
