cau hoi nay hay qua

có ai giúp tôi ko

Câu a đề không chính xác

câub) B= \(\frac{1}{1.2.3}\frac{ }{ }\)+\(\frac{1}{2.3.4}\)+\(\frac{1}{3.4.5}\)+......+\(\frac{1}{9.10.11}\)

        B= \(\frac{1}{2}\left(\frac{1}{1.2}-\frac{1}{2.3}\right)\)+.........+ \(\frac{1}{2}\left(\frac{1}{9.10}-\frac{1}{10.11}\right)\)

        B= \(\frac{1}{2}\left(\frac{1}{2}-\frac{1}{10.11}\right)\)= \(\frac{27}{55}\)

B=1/1.2.3 +1/2.3.4 +1/3.4.5 +.....+1/9.10.11

  =1/2.(2/1.2.3 +2/2.3.4 +2/3.4.5 +.......+2/9.10.11)

  =1/2.(1/1.2 -1/2.3 +1/2.3 -1/3.4 +1/4.5 +........+1/9.10 -1/10 .11)

  =1/2 .(1/1.2 -1/10.11)

  = 1/2 .27/55

  =27/110

nguyễn việt anh sai oài

thanh huyền đúng nhưng còn thiếu tí xíu

tìm thừa số chung cùng số cuối rồi sẽ tính ra

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Ta có:\(B=\dfrac{1}{6}+\dfrac{1}{24}+\dfrac{1}{60}+...+\dfrac{1}{990}\)

\(2B=\dfrac{2}{6}+\dfrac{2}{24}+\dfrac{2}{60}+...+\dfrac{2}{990}\)

\(2B=\dfrac{2}{1\cdot2\cdot3}+\dfrac{2}{2\cdot3\cdot4}+\dfrac{2}{3\cdot4\cdot5}+...+\dfrac{2}{9\cdot10\cdot11}\)

\(2B=\dfrac{1}{1\cdot2}-\dfrac{1}{2\cdot3}+\dfrac{1}{2\cdot3}-\dfrac{1}{3\cdot4}+\dfrac{1}{3\cdot4}-\dfrac{1}{4\cdot5}+...+\dfrac{1}{9\cdot10}-\dfrac{1}{10\cdot11}\)

\(2B=\dfrac{1}{1\cdot2}-\dfrac{1}{10\cdot11}\)

\(2B=\dfrac{27}{55}\)

\(B=\dfrac{27}{55}:2\)

\(B=\dfrac{27}{110}\)