alrite seems no one wants to answer this Q probably because it is too easy...
after read it a few more times I think I see it
so I just answer myself:
to see this: split LHS pk in to 1/2(pk + pk) and the first equation becomes 1/2(pk + pk)= 1/2 (pk+1 + pK-1)
rewrite above in to: 1/2(pk+1-pk) = 1/2(pk-pK-1) which is: 1/2bk+1 = 1/2bk gives us bk = bk-1