Find the sum of a probability of dice roll that is prime.

Flambergru

Flambergru

Answered question

2022-08-12

MathJax(?): Can't find handler for document MathJax(?): Can't find handler for document Find the sum of a probability of dice roll that is prime.
Consider rolling n fair dice. Let p(n) be the probability that the product of the faces is prime when you roll n dice. For example, when | { 2 , 3 , 5 } | | { 1 , 2 , 3 , 4 , 5 , 6 } | = 1 2
When | { ( 1 , 2 ) ( 2 , 1 ) , ( 1 , 3 ) , ( 3 , 1 ) ( 1 , 5 ) ( 5 , 1 ) } | | 36 | = 1 6
When p ( 3 ) = 1 24 . Find the infinite sum of p(n) (when n starts at 0) (Hint: Consider differentiating both sides of the infinite geometric series: infinite sum of 1 ( 1 r ) when ( r ) < 1)
I can differentiate the two sides of the geometric series but I'm lost regarding what to do after that. I don't fully understand the question.

Answer & Explanation

Addison Herman

Addison Herman

Beginner2022-08-13Added 15 answers

Step 1
You will get a prime product if exactly one of the faces is prime and the rest are all 1. The probability (binomial distribution) that you will have a prime product with n dice is ( n 1 ) . ( 1 2 ) 1 . ( 1 6 ) n 1
So you need to find 1 2 n = 0 n ( 1 6 ) n 1
Replace 1 6 by x. Now consider the geometric sum 1 + x + x 2 + x 3 + . . . x n + . . . hat happens when you differentiate it term by term wrt x? Especially consider what happens to the general term x n .
Step 2
Now find an expression for the geometric sum in terms of x and differentiate that wrt x. Finally substitute x = 1 6 . That should give you the required sum.

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