I have this question:
Probability distributions (joint)-screen-shot-2013-03-11-11.40.06.png
and I'm a little confused. To calculate joint distributions in the earlier questions i was using:

$\displaystyle P_{(\xi1,\xi2)}(x1,x2)=P_{(\xi1)}(x1)P_{(\xi2)}(x2 )$

But that would mean that if:

$\displaystyle P_{(\xi1,\xi2)}(2,0)=0\ either\ P_{(\xi1)}(2)=0\ or\ P_{(\xi2)}(0)=0$

which cant be true in either case as

$\displaystyle P_{(\xi1,\xi2)}(1,0)\ isnt\ 0\ and\ neither\ is\ P_{(\xi1,\xi2)}(2,1)$

Can someone please explain what I'm missing. Thanks :/