b: \(=\dfrac{3}{7}\cdot\dfrac{5}{7}\cdot\dfrac{6}{7}=\dfrac{90}{343}\)

c: \(=\dfrac{4}{9}\cdot\dfrac{5}{9}\cdot\dfrac{3}{7}=\dfrac{20}{189}\)

d: \(=\dfrac{1}{3}\cdot\dfrac{5}{12}\cdot\dfrac{5}{12}=\dfrac{25}{432}\)

d

c/      \(\frac{11.13+22.26+33.39}{22.26+44.52+66.78}=\frac{11.13+11.2.13.2+11.3.13.3}{11.2.13.2+11.2.2.13.2.2+11.2.3.13.2.3}=\frac{11.13.\left(1+2.2+3.3\right)}{11.2.13.2\left(1+2.2+3.3\right)}=\frac{11.13}{11.2.13.2}=\frac{1}{4}\)

a/7.(11+32)      7.43    301   1

  ----------------=---------=------=----

  21.(25+18)    21.43   903  3

Ta có: \(B=\frac{1}{10}+\frac{1}{15}+...+\frac{1}{120}\)

\(\Rightarrow B=\frac{2}{20}+\frac{2}{30}+...+\frac{2}{240}\)

\(\Rightarrow B=2.\left(\frac{1}{20}+\frac{1}{30}+...+\frac{1}{240}\right)\)

\(\Rightarrow B=2.\left(\frac{1}{4.5}+\frac{1}{5.6}+...+\frac{1}{15.16}\right)\)

\(\Rightarrow B=2.\left(\frac{1}{4}-\frac{1}{16}\right)\)

\(\Rightarrow B=2.\frac{3}{16}\)

\(\Rightarrow B=\frac{3}{8}\)

Vậy \(B=\frac{3}{8}\)

C=220 +230 +242 +...+2240 =2×(120 +130 +142 +...+1240 )

C=2×(14×5 +15×6 +16×7 +...+115×16 )

 2/20 + 2/30 + 2/42 + 2/56 + .... + 2/240

                   = 2/4x5 + 2/5x6 + 2/6x7 + 2/7x8 + ... + 2/15x16

                   = 2 x (1/4 - 1/5 + 1/5 - 1/6 + 1/6 - 1/7 + 1/7 - 1/8 + ... + 1/15 - 1/16)

                   = 2 x (1/4 - 1/16)

                   = 2 x 3/16 = 3/8 

k nha

Sửa đề: A=7/12-9/20+11/30-13/42+15/56

A=1/3+1/4-1/4-1/5+1/5+1/6-1/6-1/7+1/7+1/8

=1/3+1/8=11/24

\(B=\frac{5}{10.11}+\frac{5}{11.12}+\frac{5}{12.13}+...+\frac{5}{19.20}\)

\(B=5.\left(\frac{1}{10.11}+\frac{1}{11.12}+\frac{1}{12.13}+...+\frac{1}{19.20}\right)\)

\(B=5.\left(\frac{1}{10}-\frac{1}{11}+\frac{1}{11}-\frac{1}{12}+\frac{1}{12}-\frac{1}{13}+...+\frac{1}{19}-\frac{1}{20}\right)\)

\(B=5.\left(\frac{1}{10}-\frac{1}{20}\right)\)

\(B=5.\frac{1}{20}=\frac{1}{4}\)

\(C=\frac{1}{5.9}+\frac{1}{9.13}+\frac{1}{13.17}+\frac{1}{17.21}+\frac{1}{21.25}\)

\(4C=4.\left(\frac{1}{5.9}+\frac{1}{9.13}+\frac{1}{13.17}+\frac{1}{17.21}+\frac{1}{21.25}\right)\)

\(4C=\frac{4}{5.9}+\frac{4}{9.13}+\frac{4}{13.17}+\frac{4}{17.21}+\frac{4}{21.25}\)

\(4C=\frac{1}{5}-\frac{1}{9}+\frac{1}{9}-\frac{1}{13}+\frac{1}{13}-\frac{1}{15}+\frac{1}{15}-\frac{1}{17}+\frac{1}{17}-\frac{1}{21}+\frac{1}{21}-\frac{1}{25}\)

\(4C=\frac{1}{5}-\frac{1}{25}=\frac{4}{25}\)

\(C=\frac{4}{25}:4=\frac{1}{25}\)

Câu B mình giải hơi bị lỗi xíu!! Mk viết lại nhé

B=5/10.11+5/11.12+5/12.13+...+5/19.20

B=5(1/10.11+1/11.12+1/12.13+...+1/19.20)

B=5(1/10-1/11+1/11-1/12+1/12-1/13+...+1/19-1/20)

B=5( 1/10-1/20)

B=5.1/20

B=1/4

15.141+59.15=2115+885=3000

20-[30-(5-1²)]=20-[30-(5-1)]=20-[30-4)]=20-26=.....chỗ này đề bài sai nha bn

20-[30-(5-1^2)]=20-[30-(5-1)^2]=20-[30-4^2]=20-[30-16]=20-14=6