Tính : A = 1.2 + 2.3 + 3.4 + 4.5 + . . . + 2018.2019 30/07/2021 Bởi Rose Tính : A = 1.2 + 2.3 + 3.4 + 4.5 + . . . + 2018.2019
`A=1.2+2.3+3.4+4.5+…+2018.2019` `⇒3A=1.2.3+2.3.3+3.4.3+4.5.3+…+2018.2019.3` `⇒3A=1.2.3+2.3.(4-1)+3.4.(5-2)+4.5.(6-3)+…+2018.2019.(2020-2017)` `⇒3A=1.2.3+2.3.4-1.2.3+3.4.5-2.3.4+4.5.6-3.4.5+…+2018.2019.2020-2017.2018.2019` `⇒3A=2018.2019.2020` `⇒A=2018.673.2020` Bình luận
`A = 1.2 + 2.3 + 3.4 + 4.5 + . . . + 2018.2019` `⇔ 3A = 1.2.3 + 2.3.3 + 3.4.3 + 4.5.3 + … + 2018.2019.3` `⇔ 3A = 1.2.3 + 2.3.(4-1) + 3.4.(5-2) + 4.5.(6-3) + … + 2018.2019.(2020-2017)` `⇔ 3A = 1.2.3 + 2.3.4 – 1.2.3 + 3.4.5 – 2.3.4 + 4.5.6 – 3.4.5 + … + 2018.2019.2020 – 2017.2018.2019` `⇔ 3A = 2018.2019.2020` `⇔ A = 2018.673.2020` `⇔ A = 2743390280`. Bình luận
`A=1.2+2.3+3.4+4.5+…+2018.2019`
`⇒3A=1.2.3+2.3.3+3.4.3+4.5.3+…+2018.2019.3`
`⇒3A=1.2.3+2.3.(4-1)+3.4.(5-2)+4.5.(6-3)+…+2018.2019.(2020-2017)`
`⇒3A=1.2.3+2.3.4-1.2.3+3.4.5-2.3.4+4.5.6-3.4.5+…+2018.2019.2020-2017.2018.2019`
`⇒3A=2018.2019.2020`
`⇒A=2018.673.2020`
`A = 1.2 + 2.3 + 3.4 + 4.5 + . . . + 2018.2019`
`⇔ 3A = 1.2.3 + 2.3.3 + 3.4.3 + 4.5.3 + … + 2018.2019.3`
`⇔ 3A = 1.2.3 + 2.3.(4-1) + 3.4.(5-2) + 4.5.(6-3) + … + 2018.2019.(2020-2017)`
`⇔ 3A = 1.2.3 + 2.3.4 – 1.2.3 + 3.4.5 – 2.3.4 + 4.5.6 – 3.4.5 + … + 2018.2019.2020 – 2017.2018.2019`
`⇔ 3A = 2018.2019.2020`
`⇔ A = 2018.673.2020`
`⇔ A = 2743390280`.