C. 12/21

ý C

C nhà chúc bạn học tốt

b) D = \(\frac{3}{4}+\frac{3}{8}+\frac{3}{70}+\frac{3}{130}+\frac{3}{208}+\frac{3}{304}\)

    D =  \(3\left(\frac{1}{4}+\frac{1}{28}+\frac{1}{70}+\frac{1}{130}+\frac{1}{208}+\frac{1}{304}\right)\)

    D = \(3\left(\frac{1}{1x4}+\frac{1}{4x7}+\frac{1}{7x10}+\frac{1}{10x13}+\frac{1}{13x16}+\frac{1}{16x19}\right)\)

    D = \(\frac{1}{1}-\frac{1}{19}=\frac{18}{19}\)

Chắc vậy

\(F=\frac{1}{15}+\frac{1}{21}+\frac{1}{28}+...+\frac{1}{190}\)

\(\Rightarrow\)\(\frac{1}{2}F=\frac{1}{2}.\left(\frac{1}{15}+\frac{1}{21}+\frac{1}{28}+...+\frac{1}{190}\right)\)

\(\Rightarrow\) \(\frac{1}{2}F=\frac{1}{30}+\frac{1}{42}+\frac{1}{56}+...+\frac{1}{380}\)

\(\Rightarrow\)  \(\frac{1}{2}F=\frac{1}{5.6}+\frac{1}{6.7}+\frac{1}{7.8}+...+\frac{1}{19.20}\)

\(\Rightarrow\) \(\frac{1}{2}F=\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}+...+\frac{1}{19}-\frac{1}{20}\)

\(\Rightarrow\)  \(\frac{1}{2}F=\frac{1}{5}-\frac{1}{20}\)

\(\Rightarrow\) \(\frac{1}{2}F=\frac{4}{20}-\frac{1}{20}\)

\(\Rightarrow\) \(\frac{1}{2}F=\frac{3}{20}\)

\(\Rightarrow\)\(F=\frac{3}{20}\div\frac{1}{2}\)

\(\Rightarrow\) \(F=\frac{3}{20}.2\)

\(\Rightarrow\)\(F=\frac{3}{10}\)

\(F=\frac{1}{15}+\frac{ 1}{21}+...+\frac{1}{190}\)

\(F=\frac{2}{30}+\frac{2}{21}+...+\frac{2}{380}\)

\(F=\frac{2}{5.6}+...+\frac{2}{19.20}\)

\(F=2.\left(\frac{1}{5.6}+...+\frac{1}{19.20}\right)\)

\(F=2.\left(\frac{1}{5}-\frac{1}{6}+...+\frac{1}{19}-\frac{1}{20}\right)\)

\(F=2\left[\frac{1}{5}-\left(\frac{1}{6}-\frac{1}{6}\right)-...-\left(\frac{1}{19}-\frac{1}{19}\right)-\frac{1}{20}\right]\)

\(F=2.\left(\frac{1}{5}-\frac{1}{20}\right)\)

\(F=2.\frac{3}{20}\)

\(F=\frac{6}{20}=\frac{3}{10}\)

\(G=\frac{12}{84}+\frac{12}{210}+...+\frac{12}{2100}\)

\(G=\frac{4}{28}+\frac{4}{70}+...+\frac{4}{700}\)

\(G=\frac{4}{4.7}+\frac{4}{7.10}+...+\frac{4}{25.28}\)

\(G=\frac{4}{3}.\left(\frac{3}{4.7}+...+\frac{3}{25.28}\right)\)

\(G=\frac{4}{3}.\left(\frac{1}{4}-\frac{1}{7}+...+\frac{1}{25}-\frac{1}{28}\right)\)

\(G=\frac{4}{3}.\left(\frac{1}{4}-\frac{1}{28}\right)\)

\(G=\frac{4}{3}.\frac{6}{28}\)

\(G=\frac{2}{7}\)

Tổng của G và F là : \(\frac{3}{10}+\frac{2}{7}=\frac{21}{70}+\frac{20}{70}=\frac{41}{70}\)

dễ quá 

\(A=\frac{1}{1\cdot2}+\frac{1}{2\cdot3}+...+\frac{1}{49\cdot50}\)

\(A=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+...+\frac{1}{49}-\frac{1}{50}\)

\(A=1-\frac{1}{50}\)

\(A=\frac{49}{50}\)

\(A=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+...+\frac{1}{49}-\frac{1}{50}\)

\(A=1-\frac{1}{50}\)

\(A=\frac{49}{50}\)

\(A=\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{49.50}\)

\(A=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{49}-\frac{1}{50}\)

\(A=\left(1-\frac{1}{50}\right)+\left(\frac{-1}{2}+\frac{1}{2}+\frac{-1}{3}+\frac{1}{3}+...+\frac{1}{49}\right)\)

\(A=\frac{49}{50}+0\)

\(A=\frac{49}{50}\)

A = \(\frac{24}{48}\)+ \(\frac{12}{48}\)+ \(\frac{8}{48}\)+ \(\frac{2}{48}\)+ \(\frac{1}{48}\)

A = \(\frac{24+12+8+2+1}{48}\)= \(\frac{47}{48}\)

ai tốt bụng thì tk cho mk nha

tui nhìn ko có quy luật j cả

chỉ còn cách là quy đồng (ko ra quy luật)

2468/5

\(\frac{1}{10}.1234:\frac{3}{12}\)

\(=\frac{617}{5}:\frac{3}{12}\)

\(=\frac{2468}{5}\)

1/10 × 1234 : 3/12

= 1234/10 : 3/12

= 1234/10 × 4

= 2468/5.

(10000 - 47)x (72 - 47 )x (28 - 3547 - 27 )x( 48 - 48) x (73 + 6543)

kết quả =0 ,vì (48-48)=0 mà nhân 0 với bất cứ số nào thì kết quả cũng bằng 0

 bằng 3496

\(A=4\left(\frac{3}{1.4}+\frac{3}{4.7}+\frac{3}{7.10}+\frac{3}{10.13}+...+\frac{3}{94.97}\right)\)

\(A=4\left(\frac{1}{1}-\frac{1}{4}+\frac{1}{4}-\frac{1}{7}+\frac{1}{7}-\frac{1}{10}+...+\frac{1}{94}-\frac{1}{97}\right)\)

\(A=4.\left(1-\frac{1}{97}\right)=\frac{4.96}{97}=\frac{384}{97}\)

a) \(\frac{18}{35};\frac{28}{35};\frac{31}{25};\frac{32}{25}\)