`B = 2/(1.3 ) + 3/(3.6)+......+7/(21.28)`
`= 1 - 1/3 + 1/3 - 1/6 +....+1/21 - 1/28`
`=1-1/28`
`=27/28`
\(=\dfrac{1}{1}-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{6}+\dfrac{1}{6}-\dfrac{1}{10}+\dfrac{1}{10}-\dfrac{1}{15}+\dfrac{1}{15}-\dfrac{1}{21}+\dfrac{1}{21}-\dfrac{1}{28}\\ =\dfrac{1}{1}-\dfrac{1}{28}\\ =\dfrac{27}{28}\)
\(B=\dfrac{2}{1.3}+\dfrac{3}{3.6}+\dfrac{4}{6.10}+\dfrac{5}{10.15}+\dfrac{6}{15.21}+\dfrac{7}{21.28}\)
\(B=1-\dfrac{1}{3}=\dfrac{1}{3}-\dfrac{1}{6}=\dfrac{1}{6}-\dfrac{1}{10}+\dfrac{1}{10}-\dfrac{1}{15}+\dfrac{1}{15}-\dfrac{1}{21}+\dfrac{1}{21}-\dfrac{1}{28}\)
\(B=1-\dfrac{1}{28}\)
\(B=\dfrac{28}{28}-\dfrac{1}{28}\)
\(B=\dfrac{27}{28}\)
