Tarjei Knapstad wrote:
 
 > I recently ran into this problem when assisting my girlfriend 
 > with her studies for an exam in basic statistics, but I've been 
 > unable to solve it:
--snip--

Here's a hint:

If X1 and X2 are jointly normally, then: (1) y = X1-X2 is normally
distributed; and (2) the mean and standard deviation of y can be
expressed in terms of the means, standard deviations and correlation of
X1 and X2.  The problem then reduces to one of finding the probability
that |y| > a.

  Charles Metz


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