A=12/15 + 28/315

            A=8/9 

mả cha mày

2/3 + 2/15 + 2/35 + 2/63

= 2/1×3 + 2/3×5 + 2/5×7 + 2/7×9

= 1 - 1/3 + 1/3 - 1/5 + 1/5 - 1/7 + 1/7 - 1/9

= 1 - 1/9

= 8/9

\(\frac{2}{3}+\frac{2}{15}+\frac{2}{35}+\frac{2}{63}\)

= \(\frac{4}{5}+\frac{4}{45}\)

= \(\frac{8}{9}\)

2/3 + 2/15 + 2/35 + 2/63 + 2/99

= 2/1×3 + 2/3×5 + 2/5×7 + 2/7×9 + 2/9×11

= 1 - 1/3 + 1/3 - 1/5 + 1/5 - 1/7 + 1/7 - 1/9 + 1/9 - 1/11

= 1 - 1/11

= 10/11

2/3+2/15+2/35+2/63+2/99

=4/5+2/15+2/63+2/99

=14/15+2/63+2/99

=304/315+2/99

=1138/1155

\(\frac{2}{3}+\frac{2}{15}+\frac{2}{35}+\frac{2}{63}+...+\frac{2}{143}\)

\(=\frac{2}{1\times3}+\frac{2}{3\times5}+\frac{2}{5\times7}+\frac{2}{7\times9}+...+\frac{2}{11\times13}\)

\(=1-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{9}+...+\frac{1}{11}-\frac{1}{13}\)

\(=1-\frac{1}{13}=\frac{12}{13}\)

Đặt \(A=\frac{2}{3}+\frac{2}{15}+\frac{2}{35}+...+\frac{2}{143}\)

\(\Rightarrow A=\frac{2}{1\times3}+\frac{2}{3\times5}+\frac{2}{5\times7}+...+\frac{2}{11.13}\)

\(\Rightarrow A=\frac{1}{1}-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+...+\frac{1}{11}-\frac{1}{13}\)

\(\Rightarrow A=1-\frac{1}{13}=\frac{13}{13}-\frac{1}{13}=\frac{2}{13}\)

\(=\frac{2}{3}+\frac{2}{15}+\frac{2}{35}+\frac{2}{63}+\frac{2}{99}\)

\(=2.\left(\frac{1}{1.3}+\frac{1}{3.5}+\frac{1}{5.7}+\frac{1}{7.9}+\frac{1}{9.11}\right)\)

\(=2.\left(\frac{1}{1}-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+....+\frac{1}{9}-\frac{1}{11}\right)\)

\(=2.\left(\frac{1}{1}-\frac{1}{11}\right)\)

\(=\frac{20}{11}\)

\(\frac{2}{3}+\frac{2}{15}+\frac{2}{35}+\frac{2}{63}+\frac{2}{99}\)

= \(\frac{2}{1\times3}+\frac{2}{3\times5}+\frac{2}{5\times7}+\frac{2}{7\times9}+\frac{2}{9\times11}\)

= \(\frac{3-1}{1\times3}+\frac{5-3}{3\times5}+\frac{7-5}{5\times7}+\frac{9-7}{7\times9}+\frac{11-9}{9\times11}\)

= \(\frac{3}{1\times3}-\frac{1}{1\times3}+\frac{5}{3\times5}-\frac{3}{3\times5}+...+\frac{11}{9\times11}-\frac{9}{9\times11}\)

= \(1-\frac{1}{11}\)=\(\frac{10}{11}\)

\(\frac{2}{3}+\frac{2}{15}+\frac{2}{35}+\frac{2}{63}+\frac{2}{99}\)

\(=\frac{2}{1.3}+\frac{2}{3.5}+\frac{2}{5.7}+\frac{2}{7.9}+\frac{2}{9.11}\)

\(=\frac{3-1}{1.3}+\frac{5-3}{3.5}+\frac{7-5}{5.7}+\frac{9-7}{7.9}+\frac{11-9}{9.11}\)

\(=\frac{3}{1.3}-\frac{1}{1.3}+\frac{5}{3.5}-\frac{3}{3.5}+\frac{7}{5.7}-\frac{5}{5.7}+\frac{9}{7.9}-\frac{7}{7.9}+\frac{11}{9.11}-\frac{9}{9.11}\)

\(=1-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{9}+\frac{1}{9}-\frac{1}{11}\)

\(=1-\frac{1}{11}=\frac{10}{11}\)

mi tích tau tau tích mi xong tau trả lời nka việt nam nói là làm

(tính bằng máy tính)

2/3 + 2/15 + 2/35 + 2/63

= 2/1×3 + 2/3×5 + 2/5×7 + 2/7×9

= 1 - 1/3 + 1/3 - 1/5 + 1/5 - 1/7 + 1/7 - 1/9

= 1 - 1/9

= 8/9 

Ngược ngược nỗi j, cái này là TÍNH NHANH

Ủng hộ mk nha ♡_♡

a) 2/3+2/15+2/35+2/63

=2/1x3+2/3x5+2/5x7+2/7x9

=1-1/3+1/3-1/5+1/5-1/7+1/7-1/9

=1-1/9

=  8/9.

a, 2/3 + 2/15 + 2/35 + 2/63 = 8/9