离散分布的协方差

让 $X$而且 $Y$是这样的随机变量

• $P(x = 0, y = -1) = 1/5$
• $P(x = 0, y = 1) = 1/5$
• $P(x = 1, y = -1) = 1/2$
• $P(x = 1, y = 1) = 1/10$．

找到 ${X} \文本(X, Y)$．