I'm probably just missing something obvious somewhere, but what is wrong with eq. (11) on the Mathworld page? In the situation you present, A=8, x_{1}=1 and x_{2}=8 so the first term that starts with (Ax_{1} x_{2}) is identical to zero and all that remains is the second term.
Last fiddled with by FeLiNe on 20040406 at 19:20
Reason: I just learned how to do proper subscripts 'round'ere.
