\(\frac{5}{3}+\frac{5}{15}+\frac{5}{35}+\frac{5}{63}+\frac{5}{99}+\frac{5}{143}\)

\(=\frac{5}{2}\cdot\left(\frac{2}{1\cdot3}+\frac{2}{3\cdot5}+...+\frac{2}{11\cdot13}\right)\)

\(=\frac{5}{2}\cdot\left(1-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+...+\frac{1}{11}-\frac{1}{13}\right)\)

\(=\frac{5}{2}\cdot\left(1-\frac{1}{13}\right)\)

\(=\frac{5}{2}\cdot\frac{12}{13}\)

\(=\frac{30}{13}\)

\(\frac{5}{3}+\frac{5}{15}+\frac{5}{35}+\frac{5}{63}+\frac{5}{99}+\frac{5}{143}\)

\(=5\left(\frac{1}{3}+\frac{1}{15}+\frac{1}{35}+\frac{1}{63}+\frac{1}{99}+\frac{1}{143}\right)\)

\(=\frac{5}{2}\left(\frac{2}{1.3}+\frac{2}{3.5}+\frac{2}{5.7}+\frac{2}{7.9}+\frac{2}{9.11}+\frac{2}{11.13}\right)\)

\(=\frac{5}{2}\left(1-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{9}+\frac{1}{9}-\frac{1}{11}+\frac{1}{11}-\frac{1}{13}\right)\)

\(=\frac{5}{2}\left(1-\frac{1}{13}\right)\)

\(=\frac{5}{2}.\frac{12}{13}\)

\(=\frac{30}{13}\)

\(A=\frac{5}{3\cdot5}+\frac{5}{5\cdot7}+\frac{5}{7\cdot9}+......+\frac{5}{19\cdot21}\)

\(A=\frac{5}{2}\left(\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{9}+......+\frac{1}{19}-\frac{1}{21}\right)\)

\(A=\frac{5}{2}\left(\frac{1}{3}-\frac{1}{21}\right)\)

còn lại tự tính nha

ok xong r đó

\(A=\frac{5}{15}+\frac{5}{35}+\frac{5}{63}+...+\frac{5}{399}\)

\(A=\frac{5}{3.5}+\frac{5}{5.7}+\frac{5}{7.9}+...+\frac{5}{19.21}\)

\(2A=5\left(\frac{1}{3.5}+\frac{1}{5.7}+\frac{1}{7.9}+...+\frac{1}{19.21}\right)\)

\(2A=5\left(\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{9}+...+\frac{1}{19}-\frac{1}{21}\right)\)

\(2A=5\left(\frac{1}{3}-\frac{1}{21}\right)\)

\(2A=5.\frac{2}{7}\)

\(2A=\frac{10}{7}\)

\(\Rightarrow A=\frac{5}{7}\)

\(A=\frac{5}{15}+\frac{5}{35}+\frac{5}{63}+...+\frac{5}{399}\)

\(A=\frac{5}{3.5}+\frac{5}{5.7}+\frac{5}{7.9}+...+\frac{5}{19.21}\)

\(A=\frac{5.2}{3.5.2}+\frac{5.2}{5.7.2}+\frac{5.2}{7.9.2}+...+\frac{5.2}{19.21.2}\)

\(A=\frac{5}{2}\left(\frac{2}{3.5}+\frac{2}{5.7}+\frac{2}{7.9}+...+\frac{2}{19.21}\right)\)

\(A=\frac{5}{2}\left(\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{9}+...+\frac{1}{19}-\frac{1}{21}\right)\)

\(A=\frac{5}{2}\left(\frac{1}{3}-\frac{1}{21}\right)\)

\(A=\frac{5}{2}.\frac{2}{7}\)

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

Toán lớp 3?Sao giống lớp 4,5 hơn!

Ta có:

\(A=\frac{5}{15}+...+\frac{5}{399}\)

\(\Rightarrow A=5.\left(\frac{1}{3.5}+\frac{1}{5.7}+...+\frac{1}{19.21}\right)\)

\(\Rightarrow A=\frac{5}{2}.\left(\frac{1}{3}-\frac{1}{5}+...+\frac{1}{19}-\frac{1}{21}\right)\)

\(\Rightarrow A=\frac{5}{2}.\left(\frac{1}{3}-\frac{1}{21}\right)\)

\(\Rightarrow A=\frac{5}{7}\)

\(A=\frac{5}{15}+\frac{5}{35}+\frac{5}{63}+...+\frac{5}{399}\)

\(A=\frac{5}{3\cdot5}+\frac{5}{5\cdot7}+\frac{5}{7\cdot9}+...+\frac{5}{19\cdot21}\)

\(A=\frac{5}{3\cdot5\cdot2}+\frac{5}{5\cdot7\cdot2}+\frac{5}{7\cdot9\cdot2}+...+\frac{5}{19\cdot21\cdot2}\)

\(A=\frac{5}{2}\cdot\left(\frac{2}{3\cdot5}+\frac{2}{5\cdot7}+\frac{2}{7\cdot9}+...+\frac{2}{19\cdot21}\right)\)

\(A=\frac{5}{2}\cdot\left(\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{9}+...+\frac{1}{19}-\frac{1}{21}\right)\)

\(A=\frac{5}{2}\cdot\left(\frac{1}{3}-\frac{1}{21}\right)\)

\(A=\frac{5}{2}\cdot\left(\frac{7}{21}-\frac{1}{21}\right)\)

\(A=\frac{5}{2}\cdot\frac{6}{21}\)

\(A=\frac{30}{42}=\frac{5}{7}\)

cái này tính cái gì thế

ko hiểu

C .\(\frac{35}{9}=\frac{63}{5}\)

C

35.5≠63.9 do đó \(\dfrac{35}{9}\)≠\(\dfrac{63}{5}\)

=>C

TL: 

ĐÁP ÁN D 

Tick mình nha

Ta có:

    A=5/15+5/35+5/63+5/99+...+5/2915

=>A=5/3.5+5/5.7+5/7.9+5/9.11+...+5/53.55

=>A=5/2.(2/3.5+2/5.7+2/7.9+2/9.11+...+2/53.55)

=>A=5/2.(2/3-2/5+2/5-2/7+2/7-2/9+2/9-2/11+...+2/53-2/55)

=>A=5/2.(2/3-2/55)

=>A=5/2.104/165

=>A=52/33

Vậy A=52/33

OK!

thank

a) A x 3/5 = 3/1.4 + 3/4.7 + 3/7.10 + ...+ 3/100.103

               = 1 - 1/4 + 1/4 - 1/7 + 1/7 -1/10 + ... + 1/100 - 1/103

               = 1 - 1/103 

               = 102/103   => A = 102/103 : 3/5 = 170/103

b) B = 1/3.5 + 1/5.7 + 1/7.9 + ...+ 1/49.51

 B x2 = 2/3.5 + 2/5.7 + 2/7.9 + ...+ 2/49.51

         = 1/3 - 1/5 + 1/5 - 1/7 + 1/7 - 1/9 + ...+ 1.49 - 1/51

         = 1/3 - 1/51

        = 16/51

=> B = 16/51 : 2 = 8/51