>>7
　a = n + 1,
　1/n − 1/a = 1/{a(a-1)} = 1/(b-1),
　b = a(a-1) + 1,
　(1/n − 1/a) − 1/b = 1/{b(b-1)} = 1/(c-1),
　c = b(b-1) + 1,
　(1/n − 1/a − 1/b) − 1/c = 1/{c(c-1)} = 1/d,
　d = c(c-1)
　　= n(n+1)(nn+n+1)(n^4+2n^3+2n^2+n+1),