\(Taco:\frac{1}{a}-\frac{1}{a+k}=\frac{k}{a\left(a+k\right)}\)
\(B=\frac{3}{2.1}+\frac{8}{1.13}+\frac{7}{13.4}+\frac{1}{4.21}+\frac{5}{21.5}\left(dedung\right)=5\left(\frac{3}{2.5}+\frac{8}{5.13}+\frac{7}{13.20}+\frac{1}{20.21}+\frac{4}{21.25}\right)=5\left(\frac{1}{2}-\frac{1}{5}+\frac{1}{5}-.....-\frac{1}{25}\right)=5\left(\frac{1}{2}-\frac{1}{25}\right)=5.\frac{12}{25}=\frac{12}{5}\)