A mouse walks in a maze that is an orthogonal grid made of corridors that intersect at crossings one foot apart, stopping at eve
ry intersection. Using coordinates (with units in feet), it starts at the origin, then moves equally likely up to (0, 1) or down to (0, −1) or right to (1, 0) or left to (−1, 0) by one unit until the next crossing. Then it stops, picks another random direction (up, down, right or left) equally likely and moves by another unit till the next crossing. Every time it stops, its position is a vector (a, b) where both a and b are integers. Scientists let the mouse walk n feet, after which its position is (X, Y ) and it is at distance D from the origin (D = √ X2 + Y 2).
a) What is Cov(X,Y)?
b) Are X and Y independent?
c) What is E(DP)?
a) What is Cov(X,Y)?
b) Are X and Y independent?
c) What is E(DP)?
1 answer:
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First notice that the triangle with sides 
and the triangle with sides 
are similar. This is true because the angle between sides 
in the smaller triangle is clearly 
, while the angle between sides 
in the larger triangle is clearly 
. So the triangles are similar with sides corresponding to 
, respectively.
Now both triangles are
, which means there's a convenient ratio between its sides. If the length of the shortest leg is 
, then the length of the longer leg is 
and the hypotenuse has length 
.
Since
is the shortest leg in the larger triangle, it follows that 
, so 
Now both triangles are
Since