f(n)=1+1/2+1/3+…+1/2n
f(n+1)=1+1/2+1/3+…+1/2（n+1 ）= 1+1/2+1/3+…+1/(2n+2) = 1+1/2+1/3+…+1/2n+1/(2n+1)+1/(2n+2)
f(n+1)-f(n)=1/(2n+1)+1/(2n+2)