Question

1+2+3+4+5+6+7+8+9..............+2001+2002+2003+2004+2005..........+2017.............+2020=?

Answer 1

1+2+3+4+5+6+7+8+9..............+2001+2002+2003+2004+2005..........+2017.............+2020
=（1+2020）+（2+2019）+（3+2018）+....+（1010+1011）=2021*1010=204112100

Answer 2

1到2020共有2020个数,就等于有1010对数.

1+2+3+4+5+6+7+8+9..............+2001+2002+2003+2004+2005..........+2017.............+2020
=(1+2020)×(2020÷2)
=2021×1010
=204112100