Mathematics for Postgraduate Entrance Examination: addition principle, multiplication principle and permutation and combination in probability theory have been unclear. What is the probability calculation, such as taking the ball and not putting it back? Addition principle and the principle of multiplication are both simple. For example, the occurrence of event A or event B may lead to the occurrence of event C. At this time, if the probability of Pc is required, addition principle Pc=Pa+Pb must be used. If the condition of event C is that only events A and B occur at the same time, then when C occurs, Pc=Pa*Pb!

You should make clear the difference between arrangement and combination. By arrangement, I mean taking out a specified number of elements from a given number of elements and sorting them (not only taking out the specified elements, but also sorting them). Combination means that only a specified number of elements are taken out from a given number of elements, regardless of sorting (only taking out).

When arranging, combining and mixing, take a topic as an example: how many five-digit odd numbers can a non-complex number of 0, 1, 2, 3, 4 and 5 form? Because the last position and the first position have special requirements, priority should be given to preventing unqualified elements from occupying these two positions. The last number * * has C 13, then the first number * * has c, the last line * * *, and there are A34 in other positions, so these numbers should be multiplied by the principle of step-by-step counting (according to 1, this event will only happen if they appear at the same time).