= 51/128

=51/128 ha 

~HT~

Nhận xét: \(\frac{1}{5}< \frac{1}{42};\frac{1}{9}< \frac{1}{42};\frac{1}{10}< \frac{1}{42};\frac{1}{40}< \frac{1}{42}\)

\(\Rightarrow S< \frac{1}{42}+\frac{1}{42}+\frac{1}{42}+\frac{1}{42}+\frac{1}{42}\)

\(\Rightarrow S< \frac{5}{42}< \frac{21}{42}=\frac{1}{2}\)

Vậy S < 1/2

ta có: S=1159/2520 =>S<1/2

1/5+1/9+1/10+1/40+1/42=1159/2520

                      1159/2520=0.4599.....                  

                                1/2=0.5

Mà:0.5>0.4599

Nên:1/5+1/9+1/10+1/40+1/42>1/2

\(\frac{511}{1280}\)

511/1280

A = 1/5 + 1/10 + 1/20 + 1/40 + 1/80 +1/160 + 1/320 + 1/640 + 1/1280

A x 2 = 2/5 + 1/5 + 1/10 + 1/20 + 1/40 + 1/80 +1/160 + 1/320 + 1/640 

A x 2 - A = A = 2/5 - 1/1280 = 511/1280

Tham khảo ở phần Câu hỏi tương tự bạn nhé :

Câu hỏi của Trịnh Thúy An - Toán lớp 5 - Học toán với OnlineMath

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

\(B=1\cdot\frac{1}{5}+\frac{1}{2}\cdot\frac{1}{5}+\frac{1}{4}\cdot\frac{1}{5}+\frac{1}{8}\cdot\frac{1}{5}+...+\frac{1}{256}\cdot\frac{1}{5}\)

\(B=\frac{1}{5}\cdot\left(1+\frac{1}{2}+\frac{1}{4}+\frac{1}{8}+...+\frac{1}{256}\right)\)

Đặt \(A=1+\frac{1}{2}+\frac{1}{4}+\frac{1}{8}+...+\frac{1}{256}\)

\(\Rightarrow2A=2+1+\frac{1}{2}+\frac{1}{4}+...+\frac{1}{128}\)

\(\Rightarrow2A-A=2-\frac{1}{256}\)

\(A=2-\frac{1}{256}\)

Thay A vào B

có: \(B=\frac{1}{5}.\left(2-\frac{1}{256}\right)=\frac{1}{5}\cdot\frac{511}{256}=\frac{511}{1280}\)

\(B=\frac{1}{5}+\frac{1}{10}+..+\frac{1}{1280}\)

\(\Rightarrow B=\frac{1}{5}+\frac{1}{5}.\frac{1}{2}+...+\frac{1}{5}.\frac{1}{256}\)

\(\Rightarrow B=\frac{1}{5}(1+\frac{1}{2}+...+\frac{1}{256})\)

ĐẶT \(A=1+\frac{1}{2}+\frac{1}{4}+...+\frac{1}{256}\)

\(\Rightarrow2A=2+1+\frac{1}{2}+...+\frac{1}{128}\)

\(\Rightarrow2A-A=(2+1+...+\frac{1}{128})-(1+\frac{1}{2}+...+\frac{1}{256})\)

\(\Rightarrow A=2-\frac{1}{256}\)

\(\Rightarrow A=\frac{511}{256}\)

\(\Rightarrow B=\frac{511}{1280}\)

\(M=1+\dfrac{1}{5}+\dfrac{3}{35}+...+\dfrac{3}{9999}\\ =\dfrac{3}{3}+\dfrac{3}{15}+\dfrac{3}{35}+...+\dfrac{3}{9999}\\ =\dfrac{3}{2}\left(\dfrac{2}{1\cdot3}+\dfrac{2}{3\cdot5}+...+\dfrac{2}{99\cdot101}\right)\\ =\dfrac{3}{2}\left(1-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{7}+...+\dfrac{1}{99}-\dfrac{1}{101}\right)\\ =\dfrac{3}{2}\left(1-\dfrac{1}{101}\right)=\dfrac{3}{2}\cdot\dfrac{100}{101}=\dfrac{150}{101}\)

\(=\frac{63}{160}=0,39375\)

=\(=\frac{1}{5}+\frac{1}{2.5}+\frac{1}{4.5}+\frac{1}{4.8}+\frac{1}{8.5.2}+\frac{1}{8.5.4}\)

\(=\frac{1+1+1+1+1+1}{5+2.5+4.5+4.8+8.5.2+8.5}\)

\(=\frac{6}{5.8}\)

\(=\frac{6}{40}\)

mk trả lời nhanh nhất tích đúng cho mk nhé

\(A=\dfrac{1}{5}+\dfrac{1}{10}+\dfrac{1}{20}+\dfrac{1}{40}+\dfrac{1}{80}+\dfrac{1}{160}\\ A=\dfrac{35}{160}+\dfrac{16}{160}+\dfrac{8}{160}+\dfrac{4}{160}+\dfrac{2}{160}+\dfrac{1}{160}\\ A=\dfrac{66}{160}\\ A=\dfrac{33}{80}.\)

e cam on ah !

`1/5 + 1/10 + 1/20+1/40+1/80 + 1/160=A`

`<=> (32+16+8+4+2+1)/160=A`

`<=> 63/160=A`

a) 3a - 6 - 2 . (-1) = 2

    3a - 8 = 2

    3a = 10

    a = 10/3

b) 12 - a + (-7) +5 . 5 = -1

    12 - a +18 = -1

    12 - a = -19

     a = 12 - (-19)

     a = 31

c) 1 - 2 . (-3) + (-7) - 3a = -9

    0 - 3a = -9

    -3a = -9

    a = 3

a, Thay b = 6 ; c = -1 vào 3a - b - 2c = 2 

Ta có 3a - 6 - 2 x ( - 1 ) = 2

                   3a  - 6 + 2 = 2 

                              3a  = 2 + 6 - 2 

                              3a  = 6

                                a  = 2

b, Thay b = - 7 ; c = 5 vào 12 - a + b + 5c = -1

Ta có : 12 - a + ( - 7 ) + 5 x 5 = - 1 

                      12 - a - 7 + 25 = -1

                                 30 - a = -1

                                        a = 30 + 1 

                                        a = 31

c, Thay b = -3 ;c = -7 vào 1 - 2b + c - 3a = - 9

Ta có : 1 - 2 x ( -3 ) + ( -7 ) - 3a = -9

                     1 + 6 -7 - 3a = -9

                      -3a = -9

                         a = 3

a) 3a-6-2.(-1)=2

3a-8=2

3a=10

a=3/10

b) 12-a+(-7)+5

12-a+18=-19

a=12-(-19)

a=31

c) 1-2.(-3)+(-7)-3a=-9

0 -3a=-9

-3a=-9

a=3