Question

 Ti8nh tổng A= 1.2+2.3+3.4+...+2013.2014

Answer 1

a= 1.2 + 2.3 + 3.4 + 4.5 + ... + 2013.2014
3a = 1.2.3 +2.3.3 +3.4.3 +4.5.3 +... + 2013.2014.3
3a = 1.2.(3-0)+2.3(4-1)+3.4.(5-2)+...+2013.2014.(2015-2012)
3a = 1.2.3+2.3.4-1.2.3+3.4.5-2.3.4+...+2013.2014.2015-2012.2013.2014
3a=2013.2014.2015
a = 2013.2014.20153
a = 2723058910