Question

6. Tìm s

a) s - \(\frac{20}{11\times13}-\frac{20}{13\times15}-\frac{20}{15\times17}-.....-\frac{20}{53\times55}=\frac{3}{11}\)

b) \(\frac{1}{21}+\frac{1}{28}+\frac{1}{39}+...+\frac{2}{s\left(s+1\right)}=\frac{2}{9}\)

                            ( Toàn lớp 6 nha )

Answer 1

a) gt \(\Leftrightarrow\) s-\(10\times\left(\frac{2}{11\times13}+\frac{2}{13\times15}+...+\frac{2}{53\times55}\right)=\frac{3}{11}\)

\(\Leftrightarrow s-10\times\left(\frac{1}{11}-\frac{1}{13}+\frac{1}{13}-\frac{1}{15}+...+\frac{1}{53}-\frac{1}{55}\right)=\frac{3}{11}\)

\(\Leftrightarrow S-10\times\left(\frac{1}{11}-\frac{1}{55}\right)=\frac{3}{11}\)

\(\Leftrightarrow S=1\)

Answer 2

câu b hình như sai đề

Phải là \(\frac{1}{36}\) chứ ko phải \(\frac{1}{39}\)

Answer 3

ukm 1/36

Answer 4

Nhân cả 2 vế vs \(\frac{1}{2}\) có

gt \(\Leftrightarrow\frac{1}{42}+\frac{1}{56}+\frac{1}{72}+...+\frac{1}{s\left(s+1\right)}=\frac{1}{9}\)

\(\Leftrightarrow\frac{1}{6\times7}+\frac{1}{7\times8}+\frac{1}{8\times9}+...+\frac{1}{s\left(s+1\right)}=\frac{1}{9}\) 

\(\Leftrightarrow\frac{1}{6}-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}+...+\frac{1}{s}-\frac{1}{s+1}=\frac{1}{9}\)

\(\Leftrightarrow\frac{1}{6}-\frac{1}{s+1}=\frac{1}{9}\)

\(\Leftrightarrow S=17\)