Posted by Brad Paul on September 25, 2002 at 07:45:47:
In Reply to: Joint Probability posted by Rick on September 25, 2002 at 00:59:00:
: Let X and Y each take on either 1 or -1
: Let f(1,1)=P{X=1,Y=1}
: f(1,-1)=P{X=1,Y=-1}
: similarly for f(-1,1) and f(-1,-1)
: Suppose that E[X]=E[Y]=0
: show that (a) f(1,1)=f(-1,-1)
: (b) f(1,-1)=f(-1,1)
Because "E[X]=E[Y]=0" and "Let X and Y each take on either 1 or -1 " You know that P{X=1}=P{X=-1} etc. This gives you all you need.