f(n+1)=1/(n+2)+1/(n+3)+1/(n+4)+...+1/2n+1/(2n+1)+1/(2n+2)
f(n)=1/(n+1)+1/(n+2)+1/(n+3)+…+1/2n
f(n+1)-f(n)=1/(2n+1)+1/(2n+2)-1/(n+1)
=1/(2n+1)-1/(2n+2)
f(n+1)=1/(n+2)+1/(n+3)+1/(n+4)+...+1/2n+1/(2n+1)+1/(2n+2)
f(n)=1/(n+1)+1/(n+2)+1/(n+3)+…+1/2n
f(n+1)-f(n)=1/(2n+1)+1/(2n+2)-1/(n+1)
=1/(2n+1)-1/(2n+2)