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a/ Đặt 1/5= a, ta có:

1/5 + 1/10 + 1/20 + 1/40 + ... + 1/1280

= 1/a + 1/2 x a + 1/4 x a + ... + 1/256 x a
A = 1/a + 1/2 x a + 1/4 x a + ... + 1/256 x a
2 x A = 2/a + 1/a + 1/2 x a + 1/4 x a + ... + 1/128 x a
=> A = 2/a - 1/256 x a = 2/5 - 1/1280 = 511/1280

b/ 

\(\frac{121}{27}.\frac{54}{11}=\frac{11.11.27.2}{27.11}=11.2=22\)

\(\frac{100}{21}:\frac{25}{126}=\frac{100}{21}.\frac{126}{25}=\frac{25.4.21.6}{21.25}=4.6=24\)

=> \(22< n< 24\)

=> \(n=23\)

KO HIEU GIO TAY

a) \(A=\frac{1}{5}+\frac{1}{10}+\frac{1}{20}+\frac{1}{40}+...+\frac{1}{1280}\)

\(A.2=\frac{2}{5}+\frac{1}{5}+\frac{1}{10}+\frac{1}{20}+\frac{1}{40}+...+\frac{1}{640}\)

\(A.2-A=\left(\frac{2}{5}+\frac{1}{5}+\frac{1}{10}+\frac{1}{20}+\frac{1}{40}+...+\frac{1}{640}\right)-\left(\frac{1}{5}+\frac{1}{10}+\frac{1}{20}+\frac{1}{40}+...+\frac{1}{1280}\right)\)

\(A=\frac{2}{5}-\frac{1}{1280}=\frac{511}{1280}\)

b) \(\frac{121}{27}.\frac{54}{11}< n< \frac{100}{21}:\frac{25}{126}\)

\(22< n< 24\)

=>  n = 23

b) Ta có: \(\frac{121}{27}.\frac{54}{11}< n< \frac{100}{21}:\frac{25}{126}\)

\(\Leftrightarrow22< n< 24\)

\(\Rightarrow n=23\)

\(a,2448:\left[119-\left(x-6\right)\right]=24\)

\(119-\left(x-6\right)=2448:24\)

\(119-\left(x-6\right)=102\)

\(x-6=119-102\)

\(x-6=17\)

\(x=17+6\)

\(\Rightarrow x=23\)

\(b,2016-100\times\left(x+11\right)=128:8\)

\(2016-100\times\left(x+11\right)=16\)

\(100\times\left(x+11\right)=2016-16\)

\(100\times\left(x+11\right)=2000\)

\(x+11=2000:100\)

\(x+11=20\)

\(x=20-11\)

\(\Rightarrow x=9\)

\(a,2448:\left[119-\left(x-6\right)\right]=24\\ 119-\left(x-6\right)=2448:24\\ 119-\left(x-6\right)=102\\ x-6=119-102\\ x-6=17\\ x=17+6\\ x=23\\ b,2016-100\times\left(x+11\right)=128:8\\ 2016-100\times\left(x+11\right)=16\\ 100\times\left(x+11\right)=2016-16\\ 100\times\left(x+11\right)=2000\\ x+11=2000:100\\ x+11=20\\ x=20-11\\ x=9\)

a,

7x – 8 = 713

7x = 713 + 8

7x = 721

x = 721 : 7

x = 103.

b,

2448:[119-(x-6)]=24

119-(x-6)= 2448 : 24

119-(x-6)= 102

x-6= 119-102

x-6= 17

x=17+6

x=23

c,

2016 - 100.( x +11 ) = 2^7 : 2^3
2016 - 100.(x+11) = 2^4
2016 - 100.(x+11)=16
100.(x+11) = 2016-16
100.(x+11)=2000
x+11=2000:100
x+11=20
x = 20-11
x = 9

a, x=27-126

x=-99

b,\(317-\left(4x+11\right):3=20\\ 317-\left(4x+11\right)=20.3\\ 317-\left(4x+11\right)=60\\ 4x+11=317-60\\ 4x+11=257\\ 4x=257-11\\ 4x=246\\ x=61,5\)

a) x=-99
b) x=61,5

(4X+5) : 3 - 121 : 11 = 4

(4X+5) : 3 - 11 = 4

(4X+5) : 3 = 4 + 11 = 15

(4X+5) = 15 * 3 = 45

4X = 45 - 5 = 40

X = 40 : 4 = 10

Ta có: {4x+5}:3-121:11=4

{4x+5} :3- 11=4

{4x+5}:3=15

4x+5=45

4x=40

x=10

Vậy x=10.