Question

Cho mk hỏi

P=1x2+2x3+3x4+...+2017x2018=?

giải giúp mk nha

Answer 1

P=1x2+2x3+3x4+...+2017x2018

3P = 1x2x3 + 2x3x3 + 3x4x3 + ... + 2017x2018x3

3P = 1x2x3 + 2x3x(4-1) + 3x4x(5-2) + ... +2017x2018x(2019-2016)

3P = 1x2x3 + 2x3x4 - 1x2x3 + 3x4x5 - 2x3x4 + ... + 2017x2018x2019 - 2016x2017x2018

3P = 2017x2018x2019

P = 2017x2018x2019 : 3

P = 2739315938 

Answer 2

P = 1x2+2x3+3x4+...+2017x2018

3xP = 1x2x3+2x3x3+3x4x3+...+2017x2018x3

3xP = 1x2x3+2x3x(4-1)+3x4x(5-2)+...+2017x2018x(2019-2016)

3xP = 1x2x3+2x3x4-2x3x1+3x4x5-3x4x2+...+2017x2018x2019-2017x2018x2016

3xP = 2017x2018x2019

3xP = 8217947814

P = 8217947814 : 3

P = 2739315938