Tham khảo
`a)` Đặt `A=\frac{2}{60.63}+\frac{2}{63.66}+...+\frac{2}{117.120}+\frac{2}{2011}`
`⇒A=\frac{2}{3}.(\frac{3}{60.63}+\frac{3}{63.66}+...+\frac{3}{117.120})+\frac{2}{2011}`
`⇒A=\frac{2}{3}.(\frac{1}{60}-\frac{1}{63}+\frac{1}{63}-\frac{1}{66}+....+\frac{1}{117}-\frac{1}{120})+\frac{2}{2011}`
`⇒A=\frac{2}{3}.(\frac{1}{60}-\frac{1}{120})+\frac{2}{2011}`
`⇒A=\frac{2}{3}.\frac{1}{120}+\frac{2}{2011}`
`⇒A=\frac{1}{180}+\frac{2}{2011}`
`b) B=\frac{5}{40.44}+\frac{5}{44.48}+...+\frac{5}{76.80}+\frac{5}{2011}`
`⇒B=\frac{5.4}.(\frac{4}{40.44}+\frac{4}{44.48}+...+\frac{4}{76.80})+\frac{5}{2011}`
`⇒B=\frac{5}{4}.(\frac{1}{40}-\frac{1}{44}+\frac{1}{44}-\frac{1}{48}+...+\frac{1}{76}-\frac{1}{80})+\frac{5}{2011}`
`⇒B=\frac{5}{4}.(\frac{1}{40}-\frac{1}{80})+\frac{5}{2011}`
`⇒B=\frac{5}{4}.\frac{1}{80}+\frac{5}{2011}`
`⇒B=\frac{1}{64}+\frac{5}{2011}`
