Question

D = 1.2+2.3+3.4+......+2017.2018

Answer 1

D=1.2+2.3+3.4+...+2017.2018

=>3D=1.2.3+2.3.3+3.4.3+...+2017.2018.3

=>3D=1.2.(3-0)+2.3.(4-1)+3.4.(5-2)+...+2017.2018.(2019-2016)

=>3D=1.2.3-0.1.2+2.3.4-1.2.3+3.4.5-2.3.4+...+2017.2018.2019-2016.2017.2018

=>3D=\(\frac{2017.2018.2019}{3}\)

=>D=2739315938

Answer 2

Sửa lại dòng 2 từ cuối lên nhé là:

D=\(\frac{2017.2018.2019}{3}\)