Use the following inequalities to obtain an improved estimate of the sum. (Round your answers to six decimal places.)

a) sn <= ∫[1 to [infinity]] f(x) dx <= sn + 1/n
b) sn <= ∫[1 to [infinity]] f(x) dx <= sn + 1/(2n)
c) sn <= ∫[1 to [infinity]] f(x) dx <= sn + 1/(3n)
d) sn <= ∫[1 to [infinity]] f(x) dx <= sn + 1/(4n)