=3.(1/1.2+1/2.3+....+1/9.10)+11.(1/2.9+1/9.16+...+1/93.100)

=3.(1-1/2+1/2-1/3+1/3 - ... - 1/10)+11.(1/2-1/9+1/9-...-1/100)

= 3.(1-1/10) + 11.( 1/2-1/100)

= 3. 9/10+11.49/100

chỗ này bạn tự tính máy tính nha

bn ơi cho mình hỏi dấu chấm là chia à

=3

=3

= 9/10

k nha

\(\frac{1}{2\times3}+\frac{1}{3\times4}+\frac{1}{4\times5}+\frac{1}{5\times6}\)

\(=\frac{3-2}{2\times3}+\frac{4-3}{3\times4}+\frac{5-4}{4\times5}+\frac{6-5}{5\times6}\)

\(=\frac{3}{2\times3}-\frac{2}{2\times3}+\frac{4}{3\times4}-\frac{3}{3\times4}+\frac{5}{4\times5}-\frac{4}{4\times5}+\frac{6}{5\times6}-\frac{5}{5\times6}\)

\(=\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}\)

\(=\frac{1}{2}-\frac{1}{6}\)

\(=\frac{1}{3}\)

  3 / 2 / 5 + 4 / 3 / 7 - 1 / 4 + 44 / 77 - 2 / 2 / 5 - 0,75

= 17 / 5 + 31 / 7 - 1 / 4 + 4 / 7 - 12 / 5 - 3 / 4

=  ( 17 / 5 - 12 / 5 ) + ( 31 / 7 - 4 / 7 ) + ( 1 / 4 + 3 / 4 )

= 1 + 9 + 1

= 11

31/7 nha.

~học tốt~

k mình nha.

 Bài làm

=17/5 + 31/7 - 1/4 +4/7 - 12/5 - 3/10

=( 17/5+12/5)-(31/7+4/7)-3/10

=29/5-5/1-3/10

=29/5-3/10-5/1

=61/10

Mình ko chác đâu nhé!

\(A=\frac{5}{1.2}+\frac{5}{2.3}+...+\frac{5}{7.8}\)

\(\Rightarrow5A=\frac{1}{1.2}+\frac{1}{2.3}+...+\frac{1}{7.8}\)

\(\Rightarrow5A=1.\left(1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-...-\frac{1}{8}\right)\)

\(\Rightarrow5A=1-\frac{1}{8}\)

\(\Rightarrow A=\left(1-\frac{1}{8}\right).\frac{1}{5}=\frac{7}{40}\)

\(A=\frac{5}{1.2}+\frac{5}{2.3}+...+\frac{5}{7.8}\)

\(A=5\left(\frac{1}{1.2}+\frac{1}{2.3}+...+\frac{5}{7.8}\right)\)

\(A=5\left(1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+...+\frac{1}{7}-\frac{1}{8}\right)\)

\(A=5\left(1-\frac{1}{8}\right)\)

\(A=5.\frac{7}{8}\)

\(A=\frac{38}{8}\)

nhầm rồi :)

\(A=5.\left(\frac{1}{1}-\frac{1}{2}+\frac{1}{2}-....-\frac{1}{8}\right)\)

\(\Rightarrow A=\frac{5.7}{8}\)

\(\Rightarrow A=\frac{35}{8}\)

=   15

     16          

\(\frac{15}{16}\)

\(\frac{15}{16}\)

Vãi cả nhân :V

\(\frac{2}{1\cdot2}+\frac{2}{2\cdot3}+\frac{2}{3\cdot4}+\frac{2}{4\cdot5}+\frac{2}{5\cdot6}\\ =2\left(\frac{1}{1\cdot2}+\frac{1}{2\cdot3}+\frac{1}{3\cdot4}+\frac{1}{4\cdot5}+\frac{1}{5\cdot6}\right)\\ =2\left(\frac{2-1}{1\cdot2}+\frac{3-2}{2\cdot3}+\frac{4-3}{3\cdot4}+\frac{5-4}{4\cdot5}+\frac{6-5}{5\cdot6}\right)\\ =2\left(\frac{2}{1\cdot2}-\frac{1}{1\cdot2}+\frac{3}{2\cdot3}-\frac{2}{2\cdot3}+\frac{4}{3\cdot4}-\frac{3}{3\cdot4}+\frac{5}{4\cdot5}-\frac{4}{4\cdot5}+\frac{6}{5\cdot6}-\frac{5}{5\cdot6}\right)\\ =2\left(1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+...-\frac{1}{6}\right)\\ =2\left(1-\frac{1}{6}\right)\\ =2\cdot\frac{5}{6}=\frac{10}{6}\)

Chúc bạn học tốt nha.