A=1.2+2.3+3.4+4.5+5.6+...+2016.2017
=> 3A = 1.2.3+2.3.3+3.4.3+4.5.3+5.6.3+.......+2016.2017.3
=> 3A = 1.2.3 + 2.3.(4-1) + 3.4.(5-2) + 4.5.(6-3) + .......+ 2016.2017.(2018-2015)
=> 3A = 1.2.3 + 2.3.4 - 1.2.3 + 3.4.5 - 2.3.4 +..........+ 2016.2017.2018 - 2015.2016.2017
=> 3A = 2016.2017.2018
=> A = 2016.2017.2018 : 3
\(A=1.2+2.3+3.4+...+2016.2017\)
\(3A=1.2.3+2.3.\left(4-1\right)+3.4.\left(5-2\right)+...+2016.2017.\left(2018-2015\right)\)
\(3A=1.2.3+2.3.4-1.2.3+3.4.5-2.5.4+...+2016.2017.2018-2015.2016.2017\)
\(A=\frac{2016.2017.2018}{3}==2735245632\)
A=1.2+2.3+3.4+2015.2016
3A=1.2.3+2.3.3+3.4.3+...+2015.2016.3
=>3A=1.2.(3-0)+2.3.(4-1)+3.4.(5-2)+...+2015.2016.(2017-2014)
3A=1.2.3-0.1.2+2.3.4-1.2.3+3.4.5-2.3.4+...+2011.2012.2013-2014.2015.2016
3A=(1.2.3-1.2.3)+(2.3.4-2.3.4)+...+(2014.2015.2016-2014.2015.2016)+2015.2016.2017
=>3A=2015.2016.2017
A=(2015.2016.2017):3
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