1 The mistake in the question consists of five letters. If you can't remember the alphabetical order clearly, it means that you have remembered which letters, that is to say, you already know the letter r r r e o, so find the probability of misspelling this word. Then you can use an indirect method to find out the probability of writing this letter first, and then subtract this probability with 1 to get the probability of misspelling.
Probability of writing correctly: p (a) = (c11* c 31* c 21* c11)/(A55/A33)
Then the probability of wrong writing is: 1-P(A)
Question 2: There are four situations when pouring out a glass ball: C4 1 =
There are C42=6 cases when two glass balls are poured out.
C43 = There are four situations when three glass balls are poured out.
There are four cases in which C44= 1 is poured out.
* * * is 15. Because it is random, the probability of each case is115.
The probability of dumping odd-numbered grains is 1/8, and the probability of dumping even-numbered grains is 1/7, so the filling is small.
Question 3 The total number of balls S is1+2+3+...+n = ((n-1) n)/2.
The probability of drawing a ball with the number N is p (n) = n/s = 2n/(n (n-1)) = 2/(n-1).
X can be 1, 2,3 ... n ... noun (abbreviation of noun).
Expected e =1* p (1)+2 * p (2) ... n * p (n)
I have all the formulas. I'll do it myself .
Question 4: Is the answer difficult? Look at the denominator A 10, 4, 4 is the number of possible results of taking gloves. A glove *** 10 has been taken four times. Because I don't know who took which for the first time, I use a to sort it.
Let's look at the molecule, C5 1: Because there are five pairs, we should choose one of them. 2: There are two gloves, which one to take first is still uncertain. This 2 is a matter of order. A82: A takes one pair, others should consider B, and B can only take two of the remaining four pairs, that is, eight pairs, so all multiplication of A82 is the possible number of cases in which A takes one pair of gloves.
In fact, the key to thinking about permutation and combination is to take all situations into account, not to repeat or omit, and to find a formula to express this number is actually the process of selection and permutation. Explain the multiplication principle and addition principle clearly, and there will be no problem.