Nam đếm được 129 cái gì.

\(S=\dfrac{1}{2}+\dfrac{1}{2^2}+\dfrac{1}{2^3}+...+\dfrac{1}{2^{200}}\)

\(\Rightarrow2S=2\left(\dfrac{1}{2}+\dfrac{1}{2^2}+\dfrac{1}{2^3}+...+\dfrac{1}{2^{200}}\right)\)

\(\Rightarrow2S=1+\dfrac{1}{2}+\dfrac{1}{2^2}+...+\dfrac{1}{2^{199}}\)

\(\Rightarrow2S-S=\left(1+\dfrac{1}{2}+\dfrac{1}{2^2}+...+\dfrac{1}{2^{199}}\right)-\left(\dfrac{1}{2}+\dfrac{1}{2^2}+\dfrac{1}{2^3}+...+\dfrac{1}{2^{200}}\right)\)

\(\Rightarrow S=1-\dfrac{1}{2^{100}}< 1\)

\(\Rightarrow S< 1\)

Vậy \(S< 1\)

Bài 18:

a: \(\left(-\dfrac{40}{51}\cdot0,32\cdot\dfrac{17}{20}\right):\dfrac{64}{75}\)

\(=-\dfrac{40}{20}\cdot\dfrac{17}{51}\cdot\dfrac{8}{25}\cdot\dfrac{75}{64}\)

\(=-\dfrac{2}{3}\cdot\dfrac{8}{64}\cdot\dfrac{75}{25}=-\dfrac{2}{3}\cdot3\cdot\dfrac{1}{8}=-\dfrac{2}{8}=-\dfrac{1}{4}\)

b: \(-\dfrac{10}{11}\cdot\dfrac{8}{9}+\dfrac{7}{18}\cdot\dfrac{10}{11}=\dfrac{10}{11}\left(-\dfrac{8}{9}+\dfrac{7}{18}\right)\)

\(=\dfrac{10}{11}\left(-\dfrac{16}{18}+\dfrac{7}{18}\right)=\dfrac{10}{11}\cdot\dfrac{-9}{18}=\dfrac{10}{11}\cdot\dfrac{-1}{2}=-\dfrac{5}{11}\)

c: \(\dfrac{3}{14}:\dfrac{1}{28}-\dfrac{13}{21}:\dfrac{1}{28}+\dfrac{29}{42}:\dfrac{1}{28}-8\)

\(=\left(\dfrac{3}{14}-\dfrac{13}{21}+\dfrac{29}{42}\right):\dfrac{1}{28}-8\)

\(=\left(\dfrac{9}{42}-\dfrac{6}{42}+\dfrac{29}{42}\right):\dfrac{1}{28}-8\)

\(=\dfrac{32}{42}\cdot28-8=32\cdot\dfrac{2}{3}-8=\dfrac{64}{3}-\dfrac{24}{3}=\dfrac{40}{3}\)

d: \(-1\dfrac{5}{7}\cdot15+\dfrac{2}{7}\cdot\left(-15\right)+\left(-105\right)\left(\dfrac{2}{3}-\dfrac{4}{5}+\dfrac{1}{7}\right)\)

\(=-\dfrac{12}{7}\cdot15+\dfrac{2}{7}\cdot\left(-15\right)+\left(-105\right)\left(\dfrac{70}{105}-\dfrac{84}{105}+\dfrac{15}{105}\right)\)

\(=\dfrac{-180-30}{7}+\left(-105\right)\cdot\dfrac{1}{105}\)

=-30-1=-31

Bài 19:

a: \(A=7x-2x-\dfrac{2}{3}y+\dfrac{7}{9}y=5x+y\left(\dfrac{7}{9}-\dfrac{2}{3}\right)=5x+\dfrac{y}{9}\)

Khi x=-1/10;y=4,8 thì \(A=5\cdot\dfrac{-1}{10}+\dfrac{4.8}{9}\)

\(=-\dfrac{1}{2}+\dfrac{8}{15}=\dfrac{-15+16}{30}=\dfrac{1}{30}\)

b: \(B=x+\dfrac{0,2-0,375+\dfrac{5}{11}}{-0,3+\dfrac{9}{16}-\dfrac{15}{22}}\)

\(=x+\dfrac{\dfrac{2}{10}-\dfrac{6}{16}+\dfrac{10}{22}}{-\dfrac{3}{10}+\dfrac{9}{16}-\dfrac{15}{22}}=x+\dfrac{2\left(\dfrac{1}{10}-\dfrac{3}{16}+\dfrac{5}{22}\right)}{-3\left(\dfrac{1}{10}-\dfrac{3}{16}+\dfrac{5}{22}\right)}\)

\(=x-\dfrac{2}{3}\)

Khi x=-1/3 thì \(B=-\dfrac{1}{3}-\dfrac{2}{3}=-\dfrac{3}{3}=-1\)

B

- sweet-smelling: thơm, mùi hương ngọt ngào

- Dịch: Chúng là những bông hoa có mùi thơm.

Mình nghĩ là B

1 The children are watching TV at breaktime

=> The children aren't watching TV at breaktime

=> Are the children watching TV at breaktime?

2 My friend is buying a pencil sharpener

=> My friend isn't buying a pencil sharpener

=> Is your friend buying a pencil sharpener?

3 He is studying science

=> He isn't studying science

=> Is he studying science?

4 My pets are sleeping now

=> My pets aren't sleeping now

=> Are your pets sleeping now?

5 They are doing their homework

=> They aren't doing their homework

=> Are they doing their homework?

Các bạn làm giúp mình với ạ mình cảm ơn ạ

He sometimes plays golf on Sundays

The weather is always bad in November

We sometimes have fish for dinner

Peter doesn't usually get up before seven o'clock

1  He sometimes plays golf on sunday.

2  The weather is always bad in november.

3  We sometimes have fish for dinner.

4  Peter doesn't usualy get up before seven o'clock.

1a

2c

3a

4d

5c

1 A

2 C

3 A

4 D

5 C

1: \(x\in B\left(12\right)\)

=>\(x\in\left\{0;12;24;36;48;60;72;...\right\}\)

mà x<=70

nên \(x\in\left\{0;12;24;36;48;60\right\}\)

2: \(x\in B\left(8\right)\)

=>\(x\in\left\{0;8;16;24;32;40;48;56;64;...\right\}\)

mà 12<=x<=50

nên \(x\in\left\{16;24;32;40;48\right\}\)

3: \(x\in B\left(7\right)\)

=>\(x\in\left\{0;7;14;21;28;35;42;49;56;...\right\}\)

mà 16<x<56

nên \(x\in\left\{21;28;35;42;49\right\}\)

4: \(x\in B\left(5\right)\)

=>\(x\in\left\{0;5;10;15;20;25;30;35;40;...\right\}\)

mà 17<=x<=37

nên \(x\in\left\{20;25;30;35\right\}\)

Lời giải:

$23.75+25.10+25.13+180$

$=23.3.25+25.10+25.13+180$

$=25(69+10+13)+180=25.92+180=2300+180=2480$

Ngoặc không đúng rồi !

125 - 2 [ 56 - 48 : (15 - 7) ]

= 125 - 2 [ 56 - 48 : 8]

= 125 - 2 [ 56 - 6 ]

= 125 - 20. 50

= 125 - 100

= 25