https://vn.answers.yahoo.com/question/index?qid=20100907070553AAEawEd

lời giải rõ ràng rồi đó bạn ^_~

\(\frac{15-\frac{15}{7}-\frac{15}{12}}{3-\frac{3}{7}-\frac{3}{12}}\)

\(=\frac{15\left(1-\frac{1}{7}-\frac{1}{12}\right)}{3\left(1-\frac{1}{7}-\frac{1}{12}\right)}\)

\(=\frac{15}{3}\)

K  nha 

\(15-\frac{15}{7}-\frac{15}{12}=11\frac{17}{28}\)

\(=\frac{325}{28}\)

\(3-\frac{3}{7}-\frac{3}{13}=2\frac{31}{91}\)

\(=\frac{213}{91}\)

làm từng bước bạn ơi !

/ là phần

\(\frac{15-\frac{15}{7}-\frac{15}{12}}{3-\frac{3}{7}-\frac{3}{12}}\)

\(=\frac{\frac{90}{7}-\frac{15}{12}}{\frac{18}{7}-\frac{3}{12}}\)

\(=\frac{\frac{325}{28}}{\frac{65}{28}}\)

\(=\frac{35}{17}\)

\(\frac{15-\frac{15}{7}-\frac{15}{12}}{3-\frac{3}{7}-\frac{3}{12}}\)

\(=\frac{\frac{90}{7}-\frac{15}{12}}{\frac{18}{7}-\frac{3}{12}}\)

\(=\frac{\frac{325}{28}}{\frac{65}{28}}\)

\(=5\)

a: =6/12*7/14*8/16*9/18=1/2*1/2*1/2*1/2=1/16

b: =4/12*15/3*9/25*24/8

=1/3*3*5*9/25=9/5

 Đáp số : 0

\(a,=\frac{7-1}{1.3.7}+\frac{9-3}{3.7.9}+\frac{13-7}{7.9.13}+\frac{15-9}{9.13.15}\)\(+\frac{19-13}{13.15.19}\)

\(=\frac{1}{1.3}-\frac{1}{3.7}+\frac{1}{3.7}-\frac{1}{7.9}+\frac{1}{7.9}-\frac{1}{9.13}+\frac{1}{9.13}-\frac{1}{13.15}+\frac{1}{13.15}-\frac{1}{15.19}\)

\(=\frac{1}{1.3}-\frac{1}{15.19}=\frac{95}{285}-\frac{1}{285}=\frac{94}{285}\)

\(b,=\frac{1}{6}.\left(\frac{6}{1.3.7}+\frac{6}{3.7.9}+\frac{6}{7.9.13}+\frac{6}{9.13.15}+\frac{6}{13.15.19}\right)\)

làm giống như trên

\(c,=\frac{1}{8}.\left(\frac{1}{1.2.3}+\frac{1}{2.3.4}+\frac{1}{3.4.5}+...+\frac{1}{48.49.50}\right)\)

\(=\frac{1}{16}.\left(\frac{2}{1.2.3}+\frac{2}{2.3.4}+\frac{2}{3.4.5}+...+\frac{2}{48.49.50}\right)\)

\(=\frac{1}{16}.\left(\frac{3-1}{1.2.3}+\frac{4-2}{2.3.4}+\frac{5-3}{3.4.5}+...+\frac{50-48}{48.49.50}\right)\)

\(=\frac{1}{16}.\left(\frac{1}{1.2}-\frac{1}{2.3}+\frac{1}{2.3}-\frac{1}{3.4}+\frac{1}{3.4}-\frac{1}{4.5}+...+\frac{1}{48.49}-\frac{1}{49.50}\right)\)

\(=\frac{1}{16}.\left(\frac{1}{2}-\frac{1}{2450}\right)=\frac{1}{16}.\left(\frac{1225}{2450}-\frac{1}{2450}\right)=\frac{153}{4900}\)

\(d,=\frac{5}{7}.\left(\frac{7}{1.5.8}+\frac{7}{5.8.12}+\frac{7}{8.12.15}+...+\frac{7}{33.36.40}\right)\)

\(=\frac{5}{7}.\left(\frac{8-1}{1.5.8}+\frac{12-5}{5.8.12}+\frac{15-8}{8.12.15}+...+\frac{40-33}{33.36.40}\right)\)

\(=\frac{5}{7}.\left(\frac{1}{1.5}-\frac{1}{5.8}+\frac{1}{5.8}-\frac{1}{8.12}+\frac{1}{8.12}-\frac{1}{12.15}+...+\frac{1}{33.36}-\frac{1}{36.40}\right)\)

\(=\frac{5}{7}.\left(\frac{1}{5}-\frac{1}{1440}\right)=\frac{5}{7}.\left(\frac{288}{1440}-\frac{1}{1440}\right)=\frac{41}{288}\)

P/S: . là nhân nha

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