Toán 1/1.3+1/3.5+…+1/79.81 dân toán ơi :>>>>> 06/09/2021 By Valentina 1/1.3+1/3.5+…+1/79.81 dân toán ơi :>>>>>
Đặt A = $\frac{1}{1.3}$ + $\frac{1}{3.5}$ +…+ $\frac{1}{79.81}$ 2A = $\frac{2}{1.3}$ + $\frac{2}{3.5}$ +…+ $\frac{2}{79.81}$ 2A = 1 – $\frac{1}{3}$ + $\frac{1}{3}$ – $\frac{1}{5}$ + …+ $\frac{1}{79}$ – $\frac{1}{81}$ 2A = 1 – $\frac{1}{81}$ 2A = $\frac{80}{81}$ A = $\frac{80}{81}$ : 2 A = $\frac{40}{81}$ Trả lời
`1/1.3+1/3.5+…+1/79.81` `=1/2(2/1.3+2/3.5+…+2/79.81)` `=1/2(1-1/3+1/3-1/5+…+1/79-1/81)` `=1/2(1-1/81)` `=1/2 . 80/81` `=40/81` Trả lời
Đặt A = $\frac{1}{1.3}$ + $\frac{1}{3.5}$ +…+ $\frac{1}{79.81}$
2A = $\frac{2}{1.3}$ + $\frac{2}{3.5}$ +…+ $\frac{2}{79.81}$
2A = 1 – $\frac{1}{3}$ + $\frac{1}{3}$ – $\frac{1}{5}$ + …+ $\frac{1}{79}$ – $\frac{1}{81}$
2A = 1 – $\frac{1}{81}$
2A = $\frac{80}{81}$
A = $\frac{80}{81}$ : 2
A = $\frac{40}{81}$
`1/1.3+1/3.5+…+1/79.81`
`=1/2(2/1.3+2/3.5+…+2/79.81)`
`=1/2(1-1/3+1/3-1/5+…+1/79-1/81)`
`=1/2(1-1/81)`
`=1/2 . 80/81`
`=40/81`