đặt bt trên là A 

\(\frac{1}{2}\)A=\(\frac{3}{15.18}+\frac{3}{18.21}+\frac{3}{21.24}+...+\frac{3}{87.90}\)

\(\frac{1}{2}\)A=\(\frac{1}{15}-\frac{1}{18}+\frac{1}{18}-\frac{1}{21}+\frac{1}{21}-....+\frac{1}{87}-\frac{1}{90}\)

\(\frac{1}{2}A=\frac{1}{15}-\frac{1}{90}\)

..... tự tính nhé

Theo đề bài, ta có:

A = \(\dfrac{6}{15.18}+\dfrac{6}{18.21}+\dfrac{6}{21.24}+...+\dfrac{6}{87.90}\)

A = \(2.\left(\dfrac{3}{15.18}+\dfrac{3}{18.21}+\dfrac{3}{21.24}+...+\dfrac{3}{87.90}\right)\)

A =\(2.\left(\dfrac{1}{15}-\dfrac{1}{18}+\dfrac{1}{18}-\dfrac{1}{21}+\dfrac{1}{21}-\dfrac{1}{24}+...+\dfrac{1}{87}-\dfrac{1}{90}\right)\)

A = \(2.\left(\dfrac{1}{15}-\dfrac{1}{90}\right)\)= \(\dfrac{1}{8}\)

A= \(\dfrac{6}{15.18}+\dfrac{6}{18.21}+...+\dfrac{6}{87.90}\)

A= \(2\left(\dfrac{3}{15.18}+\dfrac{3}{18.21}+...+\dfrac{3}{87.90}\right)\)

A= \(2\left(\dfrac{1}{15}-\dfrac{1}{18}+\dfrac{1}{18}-\dfrac{1}{21}+...+\dfrac{1}{87}-\dfrac{1}{90}\right)\)

A= \(2\left(\dfrac{1}{15}-\dfrac{1}{90}\right)\)

A= 2. \(\dfrac{1}{16}\)

A= \(\dfrac{1}{8}\)

A =\(\dfrac{6}{15.18}+\dfrac{6}{18.21}+\dfrac{6}{21.24}+...+\dfrac{6}{87.90}\)

= \(\dfrac{6}{3}.\left(\dfrac{3}{15.18}+\dfrac{3}{18.21}+\dfrac{3}{21.14}+...+\dfrac{3}{87.90}\right)\)

=\(\dfrac{6}{3}.\left(\dfrac{1}{15}-\dfrac{1}{18}+\dfrac{1}{18}-\dfrac{1}{21}+\dfrac{1}{21}-\dfrac{1}{24}+...+\dfrac{1}{87}+\dfrac{1}{90}\right)\)

=\(\dfrac{6}{3}.\left(\dfrac{1}{15}-\dfrac{1}{90}\right)\)

= \(\dfrac{6}{3}.\dfrac{1}{18}=\dfrac{1}{9}\)

a) A = 1/3 - 1/7 + 1/7 - 1/11 +......+1/107 - 1/111

A = 1/3 - 1/111

A = ..............Bạn tự tính nhé!

b) B = 2.(3/15.18 + 3/18.21 +........+3/87.90)

B = 2.(1/15 - 1/18 + 1/18 - 1/21 +........+1/87 - 1/90)

B = 2.(1/15 - 1/90)

B = 2.5/90

B =......Tự tính nhé!

C ; D làm tương tự nhé!

yêu cầu là gì vậy

a)

A=\(\frac{4}{3.7}+\frac{4}{7.11}+....+\frac{4}{107.111}=\frac{1}{3}-\frac{1}{7}+\frac{1}{7}-.....+\frac{1}{107}-\frac{1}{111}=\frac{1}{3}-\frac{1}{111}=\frac{108}{333}\)

\(A=2\left(\frac{1}{15}-\frac{1}{18}+\frac{1}{18}-\frac{1}{21}+......+\frac{1}{87}-\frac{1}{90}\right)\)

\(\Rightarrow A=2\left(\frac{1}{15}-\frac{1}{90}\right)\)

\(\Rightarrow A=2.\frac{1}{18}=\frac{1}{9}\)

\(A=\frac{6}{15.18}+\frac{6}{18.21}+\frac{6}{21.24}+...+\frac{6}{87.90}\)

\(=6.\frac{1}{3}.\left(\frac{3}{15.18}+\frac{3}{18.21}+\frac{3}{21.24}+...+\frac{3}{87.90}\right)\)

\(=2.\left(\frac{1}{15}-\frac{1}{18}+\frac{1}{18}-\frac{1}{21}+\frac{1}{21}-\frac{1}{24}+...+\frac{1}{87}-\frac{1}{90}\right)\)

\(=2.\left(\frac{1}{15}-\frac{1}{90}\right)\)

\(=2.\frac{1}{18}\)

\(=\frac{1}{9}\)

\(B=\frac{1}{10}+\frac{1}{15}+\frac{1}{21}+...+\frac{1}{120}\)

\(=\frac{2}{20}+\frac{2}{30}+\frac{2}{42}+...+\frac{2}{240}\)

\(=\frac{2}{4.5}+\frac{2}{5.6}+\frac{2}{5.7}+...+\frac{2}{15.16}\)

\(=2.\left(\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}+...+\frac{1}{15.16}\right)\)

\(=2.\left(\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+...+\frac{1}{15}-\frac{1}{16}\right)\)

\(=2.\left(\frac{1}{4}-\frac{1}{16}\right)\)

\(=2.\frac{3}{16}\)

\(=\frac{3}{8}\)

a) \(A=\frac{6}{15.18}+\frac{6}{18.21}+...+\frac{6}{87.90}\)

\(A=\frac{3.6}{3.15.18}+\frac{3.6}{3.18.21}+...+\frac{3.6}{3.87.90}\)

\(A=2\left(\frac{3}{15.18}+\frac{3}{18.21}+...+\frac{3}{87.90}\right)\)

\(A=2\left(\frac{1}{15}-\frac{1}{18}+\frac{1}{18}-\frac{1}{21}+...+\frac{1}{87}-\frac{1}{90}\right)\)

\(A=2\left(\frac{1}{15}-\frac{1}{90}\right)\)

\(A=2.\frac{1}{18}=\frac{1}{9}\)

(*Không chép lại A nhé, mỏi tay :V*)

\(A=\frac{21-18}{18\cdot21}+\frac{24-21}{21\cdot24}+...+\frac{90-87}{87\cdot90}\\ =\frac{1}{18}-\frac{1}{21}+\frac{1}{21}-\frac{1}{24}+...+\frac{1}{87}-\frac{1}{90}\\ =\frac{1}{18}-\frac{1}{90}=\frac{2}{45}\)

Ez, chúc bạn học tốt nha.

\(B=\frac{5}{18\cdot21}+\frac{5}{21\cdot24}+\frac{5}{24\cdot27}+...+\frac{5}{123\cdot126}\\ B=\frac{5}{3}\cdot\left(\frac{3}{18\cdot21}+\frac{3}{21\cdot24}+\frac{3}{24\cdot27}+...+\frac{3}{123\cdot126}\right)\\ B=\frac{5}{3}\cdot\left(\frac{1}{18}-\frac{1}{21}+\frac{1}{21}-\frac{1}{24}+\frac{1}{24}-\frac{1}{27}+...+\frac{1}{123}-\frac{1}{126}\right)\\ B=\frac{5}{3}\cdot\left(\frac{1}{18}-\frac{1}{126}\right)\\ B=\frac{5}{3}\cdot\left(\frac{7}{126}-\frac{1}{126}\right)\\ B=\frac{5}{3}\cdot\frac{1}{21}\\ B=\frac{5}{63}\)

Đây là những bài lớp 5+ vậy nên không làm theo kiểu lớp 5 được, cố gắng tìm bài đúng lớp nhé bạn (VNNLL cc)

A= 1/18 nhé bạn

xin lỗi vì mình không viết cách làm..........