def sum lo, hi, &term
  (lo..hi).sum(&term)
end
p sum(1,100){|i| 1.0/i}   # => 5.187377517639621
# or using Rational:
p sum(1,100){|i| Rational(1,i)}  # => 14466636279520351160221518043104131447711 / 2788815009188499086581352357412492142272