Update Probability.md

Probability.md

@@ -61,3 +61,8 @@
 * **Proposition 4.4**
 
     ![](https://latex.codecogs.com/gif.latex?P(\bigcup_{1}^{n}E_{i})&space;=&space;\sum_{1}^{n}P(E_{i})-\sum_{i_{1}%3Ci_{2}}P(E_{i_{1}}E_{i_{2}})+...&space;+(-1)^{r+1}\sum_{i_{1}%3Ci_{2}%3C...%3Ci_{r}}P(E_{i_{1}}E_{i_2}...E_{i_{r}})+...+(-1)^{n+1}P(E_{i_{1}}E_{i_{2}}...E_{i_{n}}))))
+
+    In words, Proposition 4.4 states that the probability of the union of n events equals
+the sum of the probabilities of these events taken one at a time, minus the sum of the
+probabilities of these events taken two at a time, plus the sum of the probabilities of
+these events taken three at a time, and so on.