- #1
RsMath
- 7
- 0
I have these two question I hope someone could check them out for me :
1) A die is rolled 3 times, let x be the sum of the 3 results .
what is var(x) ?
I started by calculating E(x)= Segma(1 to 3 (E(Xi)) when Xi is the number we got in every throw .
E(Xi)=3.5
then E(x) = 10.5 (Expected of the sum)
now what should I do to find the var of x ?
2) we have n balls and n^2 chains , we throw the balls randomly into the chains.
we pick y randomly - a chain , what's the probability that it has exactly 3 balls in it .
ok, I've kinda solve this question but I'm not sure if the answer is correct .. we choose 3 balls from the n balls , and we do it in n!/((n-3)!*3!) and we calculate every ball by the probability that it goes into y , 1/n^2 .. thus final answer is :
n!
-------------
(n-3)!*3!*n^6
can you help me complete the first Q and check the second one for me.
thanks.
1) A die is rolled 3 times, let x be the sum of the 3 results .
what is var(x) ?
I started by calculating E(x)= Segma(1 to 3 (E(Xi)) when Xi is the number we got in every throw .
E(Xi)=3.5
then E(x) = 10.5 (Expected of the sum)
now what should I do to find the var of x ?
2) we have n balls and n^2 chains , we throw the balls randomly into the chains.
we pick y randomly - a chain , what's the probability that it has exactly 3 balls in it .
ok, I've kinda solve this question but I'm not sure if the answer is correct .. we choose 3 balls from the n balls , and we do it in n!/((n-3)!*3!) and we calculate every ball by the probability that it goes into y , 1/n^2 .. thus final answer is :
n!
-------------
(n-3)!*3!*n^6
can you help me complete the first Q and check the second one for me.
thanks.