# Summary 3¶

In this section we learnt how to build **probability trees** and how to use two rules to calculate probabilities of events combined by 'ANDs' or 'ORs' in the following way:

**the product rule**: $P(A \ and \ B)=P(A)\times P(B)$,**the addition rule**: $P(A \ or \ B)=P(A)+P(B)$.

We've said that both of these rules have conditions that need to be met in order to use them, but we'll go into more detail in future sections.

We've also learnt that we can use the rules several times and without the need of the probability tree, which allow us to calculate longer and more complicated probabilities.

Finally, we compared some *theoretical probabilities* that we now know how to calculate with the *empirical probability* of modeling the experiment and getting the relative frequency.