from binomial coefficient formula Attribute eq6-3 to eq6-2 - $\sum_i \binom{A + i}{i}\binom{B + K - i}{K - i}$ - $= \sum_i \binom{A + i}{A}\binom{B + K - i}{B}$ ... eq4-3 - $= \sum_j \binom{j}{A}\binom{B + K - j + A}{B}$ ... A+i -> j - $= \sum_j \binom{j}{A}\binom{N - j -1}{B}$ ... A+B+K+1 -> N - $= \binom{N}{A + B + 1}$ ... eq6-2 - $= \binom{A+B+K+1}{A + B + 1} = \binom{A+B+K+1}{K}$
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