Question

Tính: S = 1+2-3-4+5+6-7-8+9+10+ ..... + 2018-2019-2020+2021

Answer 1

Ta có: \(S=1+2-3-4+5+6-...+2018-2019-2020+2021\)

\(=\left(-4\right)\cdot505+2021\)

=2021-2020

=1

Answer 2

\(S=\left(1+2-3-4\right)+\left(5+6-7-8\right)+...+\left(2017+2018-2019-2020\right)+2021\\ S=\left(-4\right)+\left(-4\right)+...+\left(-4\right)+2021\)

Ta có từ 1 đến 2020 có 2020 số nên khi nhóm 4 số 1 cặp thì có \(2020:5=404\left(cặp\right)\)

Vậy \(S=404\left(-4\right)+2021=-1616+2021=405\)

Answer 3

S=1+(2-3)+(-4+5)+(6-7)+(-8+9)+...+(-2020+2021)
S=1-1+1-1+1+...+1
S=1+0+0+...+0
S=1

Answer 4

S = 1 + 2 - 3 - 4 + 5 + 6 - 7 - 8 + ... + 2018 – 2019 - 2020 + 2021 

 S = (1 + 2 - 3 - 4) + ... + (2017 + 2018 – 2019 - 2020) + 2021 S = (-4) + ... + (-4) + 2021 +  2020 : 4 = 505

Answer 5

S = 1 + 2 - 3 - 4 + 5 + 6 - 7 - 8 + ... + 2018 – 2019 - 2020 + 2021 

 S = (1 + 2 - 3 - 4) + ... + (2017 + 2018 – 2019 - 2020) + 2021 S = (-4) + ... + (-4) + 2021 +  2020 : 4 = 505 S = (-4) . 505 + 2021  S = (-2020) + 2021  S = 1