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whitejac
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Homework Statement
For reference, this is the image setting up the problem.
"A wireless sensor grid consists of 21×11=231 sensor nodes that are located at points (i,j) in the plane such that i∈{0,1,⋯,20} and j∈{0,1,2,⋯,10} as shown in Figure 2.1. The sensor node located at point (0,0) needs to send a message to a node located at (20,10). The messages are sent to the destination by going from each sensor to a neighboring sensor located above or to the right. That is, we assume that each node located at point (i,j) will only send messages to the nodes located at (i+1,j) or (i,j+1). How many different paths do exist for sending the message from node (0,0) to node (20,10)?"
Problem 11
In Problem 10, assume that all the appropriate paths are equally likely. What is the probability that the sensor located at point (10,5) receives the message? That is, what is the probability that a randomly chosen path from(0,0) to (20,10) goes through the point (10,5)?
Problem 12
In Problem 10, assume that if a sensor has a choice, it will send the message to the above sensor with probability pa and will send the message to the sensor to the right with probability pr=1−pa. What is the probability that the sensor located at point (10,5) receives the message
Homework Equations
C(n k) = n! / k!(n-k)!
P(A) = |A| / |S|
P(A) = C(n k)(Pan)((1-(Pa))n-k
The Attempt at a Solution
This is an extension of a previous homework probleem i did.
I know that the answer to problem 10 is:
C(30 20)
So this is 'given.' The justification by the professor was that you have 30 necessary routes (20 to the right and ten to the top) and so you have to choose 20 of them (or ten of them) and subsequently you will get the remaining segment as well.
Problem 11 is a simple rendition of this one. I worked it out with my friend and we concluded fairly confidently that if you had 30 routes and needed to know the probability of getting it through the middle then you would have to a separate cardinality of C(15 10). This would grant you all of the possible ways to get to the midddle sensor. Using the probability theorem,
P(A) = |A|/|S| = C(10 ) / C(30 20) ≈ 9.99e-5
Problem 12 is the catch. We did not know where to even go with this one. We found some answers that were basically way too large for what we thought we were searching for.
This was my solution, derived from the binomial theorem where i have n bernoulli trials and k successes, where a success is given by a shift to the right so that my exponents maintained the direction i intended to go:
P(A) = C(15 10)(0.510)(0.55)
The point five was to mirror the results from problem 11. I called these bernoulli trials because each sensor was independant of the previous one for k < 20, because obviously once it reaches the edges it no longer has a choice. This yieled an answer different from the result of problem 11, and my friend who REALLY enjoys math could not figure this out. I believe the flaw in my answer is that it's an inappropriate rendition of the bernoulli trials, and thus the binomial theorem does not count. However, my friend began charting out the probability of reaching each sensor independantly and it seemed like that was the right direction to head towards. So I'm at a loss.
My professor said to study from the book for the exam and that his goal was to make everybody make a 50 on his tests. So something like this showing up on my intro to stats class is very real fear of mine. If anyone could shed some light on this type of counting method, I'd be greatly appreciative.