\(\frac{8}{35}:\frac{-3}{\frac{9}{20}}=\frac{8}{35}:\frac{-20}{3}=\frac{8}{35}.\frac{3}{-20}=\frac{-6}{175}\)

\(A=\frac{1}{2}+\frac{1}{2^2}+\frac{1}{2^3}+...+\frac{1}{2^{20}}\)

\(\Rightarrow2A=1+\frac{1}{2}+\frac{1}{2^2}+...+\frac{1}{2^{19}}\)

\(\Rightarrow2A-A=(1+\frac{1}{2}+\frac{1}{2^2}+...+\frac{1}{2^{19}})-(\frac{1}{2}+\frac{1}{2^2}+...+\frac{1}{2^{20}})\)

\(\Rightarrow A=1-\frac{1}{2^{20}}< 1\)

ta có:B= \(\frac{10^8}{10^8}\)-3=1-3=-2<0

mà A=10⁸-1+\(\frac{2}{10^8}\)>0

=> A>B

\(A=\frac{10^8+2}{10^8-1}=\frac{10^8-1+3}{10^8-1}=1+\frac{3}{10^8-1}=1\frac{3}{10^8-1}\)

\(B=\frac{10^8}{10^8-3}=\frac{10^8-3+3}{10^8-3}=1+\frac{3}{10^8-3}=1\frac{3}{10^8-3}\)

Mà \(1\frac{3}{10^8-1}< 1\frac{3}{10^8-3}\Rightarrow A< B\)

\(25\%=\frac{1}{4}\)

Suy ra \(A=\frac{1}{4}-\frac{5}{4}-\frac{1}{\frac{5}{6}}=-1-\frac{6}{5}=-\frac{11}{5}\)

Để \(A=\frac{2n-1}{3n-1}\inℤ\)

\(\Rightarrow3A=\frac{3\left(2n-1\right)}{3n-1}=\frac{6n-3}{3n-1}\inℤ\)

\(\Rightarrow3A=\frac{2\left(3n-1\right)}{3n-1}=\frac{2\left(3n-1\right)}{3n-1}=1-\frac{1}{3n-1}\)

\(\Rightarrow3A=2-\frac{1}{3n-1}\)

Do \(2\inℤ\)nên để \(3A=2\)thì \(\frac{1}{3n-1}=2\)

\(\Rightarrow3n-1\inƯ\left(1\right)=\left\{\pm1\right\}\)

Lập bảng :

Gía trị \(\frac{2}{3}\notinℤ\)(loại)

Với n=0 \(\Rightarrow A=\frac{2.0-1}{3.0-1}=\frac{-1}{-1}=1\)

\(\Rightarrow A=1\left(\inℤ\right)\)

Vậy với n=0 thì phân số \(A=\frac{2n-1}{3n-1}=1\)có giá trị là số nguyên 

Để A nguyên \(\Rightarrow\frac{2n-1}{3n-1}\inℤ\)

\(\Rightarrow2n-1⋮3n-1\)

\(\Rightarrow3\left(2n-1\right)-2\left(3n-1\right)⋮3n-1\)

\(\Rightarrow6n-3-6n+2⋮3n-1\)

\(\Rightarrow-1⋮3n-1\)

\(\Rightarrow3n-1\inƯ\left(-1\right)\)

\(\Rightarrow3n-1\in\left\{-1;1\right\}\)mà 3n-1 chia 3 dư 2

\(\Rightarrow3n-1=-1\)

\(\Rightarrow n=0\)

Ta có \(\frac{91}{1.4}+\frac{91}{4.7}+\frac{91}{7.10}+...+\frac{91}{88.91}=\frac{91}{3}.\left(\frac{3}{1.4}+\frac{3}{4.7}+\frac{3}{7.10}+...+\frac{3}{88.91}\right)\)

\(=\frac{91}{3}.\left(1-\frac{1}{4}+\frac{1}{4}-\frac{1}{7}+\frac{1}{7}-\frac{1}{10}+...+\frac{1}{88}-\frac{1}{91}\right)\)

\(=\frac{91}{3}.\left(1-\frac{1}{91}\right)=\frac{91}{3}.\frac{90}{91}=30\)

9/1.4+9/4.7+9/7.10+…+9/88.91=30

=91/3. (3/1.4+3/4.7+3/7.10+…+3/88.91)

=91/3. (1-1/4+1/4-1/7+1/7-1/10+…+1/88-1/91)

=91/3*90/91=30

Mn giúp mk câu này đi cần gấp

mn ơi giúp mk vsss

PÉ x 0 kia kìa

thặc dễ thương

=0 vì mọi số x0 đều =0