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A=1.2+2.3+3.4+4.5+...+2014.2015

=>3A=1.2.3+2.3.3+3.4.3+4.5.3+...+2014.2015.3

=1.2.(3-0)+2.3.(4-1)+3.4.(5-2)+4.5.(6-3)+...+2014.2015.(2016-2013)

=1.2.3-0.1.2+2.3.4-1.2.3+3.4.5-2.3.4+4.5.6-3.4.5+...+2014.2015.2016-2013.2014.2015

=(1.2.3-1.2.3)+(2.3.4-2.3.4)+(3.4.5-3.4.5)+(4.5.6-4.5.6)+...+(2013.2014.2015-2013.2014.2015)+0.1.2+2014.2015.2016

=0+2014.2015.2016

=>A=\(\frac{2014.2015.2016}{3}\)

S=9/10

bằng 9/10 đó bạn

S=\(\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+.....+\frac{1}{9.10}\)

S=\(1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{9}-\frac{1}{10}\)

S=\(\frac{1}{10}-1\)

S=\(\frac{9}{10}\)

\(S=\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{2011.2012}\)

\(S=\frac{1}{1}-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{2011}-\frac{1}{2012}\)

\(S=1-\frac{1}{2012}\)

\(S=\frac{2011}{2012}\)

Chúc bạn học tốt nha !!!

=1-1/2+1/2-1/3+1/3-1/4+...+1/2011-1/2012

= 1-1/2012

= 2011/2012

\(S=\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{2011.2012}\)

\(\Rightarrow S=\frac{1}{1}-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{2011}-\frac{1}{2012}\)

\(\Rightarrow S=1-\frac{1}{2012}=\frac{2011}{2012}\)

\(=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{2014}-\frac{1}{2015}\)

\(=1-\frac{1}{2015}\)

\(=\frac{2014}{2015}\)

A=1/1-1/2+1/2-1/3+1/3-...........+1/2014-1/2015

A=1/1-1/2015

A=2014/2015

cau hỏi tương tự ko có mà!!!!!!!!!!!!!!!!!!!!!!!!!!!!

3C=1.2.3+2.3.(4-1)+3.4.(5-2)+...+2014.2015.(2016-2013)

3C=2014.2015.2016

C=2014.2015.2016:3

S=2-1/1.2  .  3-2/2.3............2016-2015/2015.2016

  =1/1 - 1/2 + 1/2 - 1/3+........+1/2015 - 1/2016

  =1/1 - 1/2016

   =2015/2016

\(=\frac{0}{1.2}+\frac{0}{2.3}+\frac{0}{3.4}+...+\frac{0}{2015.2016}\)

\(=0+0+0+...+0=0\)