Generally speaking, the probability is level one and level two; Rank and combine two levels, the probability is divided into two parts.

Probability: textbook, compulsory 3, elective 2-3.

Arrangement and combination: 2-3 textbooks.

Probability, also known as "probability", reflects the probability of random events. Random events refer to events that may or may not occur under the same conditions.

For example, it is a random event to randomly select one from a batch of goods with genuine products and defective products, and "what is extracted is genuine products".

Suppose a random phenomenon is tested and observed n times, in which event A appears m times, that is, its frequency is m/n. After a lot of repeated experiments, m/n is often closer to a constant (see Bernoulli's law of large numbers for details).

This constant is the probability of the occurrence of event A, which is usually expressed by P (A).