\(A=\left(1-\frac{1}{15}\right)\left(1-\frac{1}{21}\right)\left(1-\frac{1}{28}\right)...\left(1-\frac{1}{79800}\right)\)

\(A=\frac{14}{15}.\frac{20}{21}.\frac{27}{28}...\frac{209}{210}\)

\(A=\frac{28}{30}.\frac{40}{42}.\frac{54}{56}...\frac{418}{240}\)

\(A=\frac{4.7}{5.6}.\frac{5.8}{6.7}.\frac{6.9}{7.8}...\frac{19.22}{20.21}\)

\(A=\frac{4.5.6...19}{5.6.7...20}.\frac{7.8.9...22}{6.7.8...21}\)

\(A=\frac{4}{20}.\frac{22}{6}\)

\(A=\frac{11}{15}\)

= 14/15 x20/21 x 27/28 x ... x 209/210

= 28/30 x 40/42 x 54/56 x ... x 418/420

= 4x7/5x6 x 5x8/6x7 x 6x9/7x8 x ... x 19x22/20x21

= (4x7x5x8x6x9x...x19x22) / (5x6x6x7x7x8x...x20x21)

bạn gộp vào và tối giản đi, cuối cùng đc : 11/15 ( mik ko chắc chắn đâu nên bạn thử lại nhé)

thx bạn Ngô Khánh Linh

\(\left(1-\frac{1}{15}\right)\left(1-\frac{1}{21}\right)\left(1-\frac{1}{28}\right)....\left(1-\frac{1}{210}\right)\)

\(=\frac{14}{15}\cdot\frac{20}{21}\cdot\frac{27}{28}\cdot.....\cdot\frac{209}{210}\)

\(=\frac{28}{30}\cdot\frac{40}{42}\cdot\frac{54}{56}\cdot....\cdot\frac{418}{420}\)

\(=\frac{4\cdot7}{5\cdot6}\cdot\frac{5\cdot8}{6\cdot7}\cdot\frac{6\cdot9}{7\cdot8}\cdot...\cdot\frac{19\cdot22}{20\cdot21}\)

\(=\frac{4\cdot5\cdot6\cdot...\cdot19}{5\cdot6\cdot7\cdot8\cdot20}\cdot\frac{7\cdot8\cdot9\cdot....\cdot22}{6\cdot7\cdot8\cdot....\cdot21}\)

\(=\frac{4}{20}\cdot\frac{22}{6}=\frac{11}{15}\)

giuos mình nhé . mình cần khẩn cấp đó các bạn ơi