The sum of the probabilities of all of the possible events should be equal to 1.Take the coin and flip it twice. What is probability to get twice head?There are 4 situation, 1H and 1H, 1H and 1T, 1T and 1H, 1T and 1T.Then there are 1/4 chance to get the conin and flip it twice get both head.Every flip is an independent event.The both independent probability can * each other make what is probabilities to get both situation related.Probabilities to get the T first, H second, T third is each independent probabilities can *. So it is 1/2*1/2*1/2=1/8.Every probabilities is an INDEPENDENT event.
Probability of A or B=Probability of A plus Probability of B and minus Probability both A and B with overlap.Mutually Exclusive, that they both can't happen at the same t
得到一个数1的概率表示为P(1) = 1/6
:)