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
\(\frac{1}{1x2x3}+\frac{1}{2x3x4}+\frac{1}{3x4x5}+....+\frac{1}{9x10x11}\)
B=\(\frac{1}{2}x\orbr{\begin{cases}1\\1x2\end{cases}-\frac{1}{2x3}+\frac{1}{2x3}-\frac{1}{3x4}+...+\frac{1}{9x10}-\frac{1}{10x11}}\)
B=\(\frac{1}{2}x\orbr{\begin{cases}1\\1x2\end{cases}}-\frac{1}{10x11}\)
B=\(\frac{1}{2}x\frac{27}{55}\)
B=\(\frac{27}{110}\)
