`sao ko dùng nick kia, sang nick này lmj`

C

ngta ghi nguồn đàng hoàng mà bảo xuống, tham lam

` 3xx 7/10 + 7/10 xx 5 +2 xx 7/10`

`=7/10 xx (3+5+2)`

`= 7/10 xx 10`

`=7`

Integer

`x xx 3/4=1/5`

`x=1/5:3/4`

`x=4/15`

B

\(\dfrac{x}{4}=\dfrac{9}{x}\\ \Rightarrow x.x=9.4\\ \Rightarrow x^2=36\\ \Rightarrow x^2=\left(\pm6\right)^2\\ \Rightarrow x=\pm6\)

\(\dfrac{1}{20}+\dfrac{1}{44}+\dfrac{1}{77}+...+\dfrac{2}{x\left(x+3\right)}=\dfrac{101}{770}\\ \Leftrightarrow\dfrac{3}{2}\left(\dfrac{1}{20}+\dfrac{1}{44}+\dfrac{1}{77}+...+\dfrac{2}{x\left(x+3\right)}\right)=\dfrac{3}{2}.\dfrac{101}{770}\\ \Leftrightarrow\dfrac{3}{40}+\dfrac{3}{88}+\dfrac{3}{154}+...+\dfrac{3}{x\left(x+3\right)}=\dfrac{303}{1540}\\ \Leftrightarrow\dfrac{3}{5.8}+\dfrac{3}{8.11}+\dfrac{3}{11.14}+...+\dfrac{3}{x\left(x+3\right)}=\dfrac{303}{1540}\\ \Leftrightarrow\dfrac{1}{5}-\dfrac{1}{8}+\dfrac{1}{8}-\dfrac{1}{11}+...+\dfrac{1}{x}-\dfrac{1}{x+3}=\dfrac{303}{1540}\\ \Leftrightarrow\dfrac{1}{5}-\dfrac{1}{x+3}=\dfrac{303}{1540}\)

\(\Rightarrow\dfrac{1}{x+3}=\dfrac{1}{308}\\ \Leftrightarrow x+3=308\\ \Leftrightarrow x=305\)