\(\left(x\cdot2,4-4,2\right)\div x=1\)
\(\Rightarrow x\cdot2,4-4,2=x\)
\(\Rightarrow x\cdot2,4=x+4,2\)
\(\Rightarrow\frac{12x}{5}=\frac{5x+21}{5}\)
\(\Rightarrow12x=5x+21\)
\(\Rightarrow12x-5x=21\)
\(\Rightarrow7x=21\Rightarrow x=\frac{21}{7}=3\)
Vậy x = 3
\(\frac{1}{1\cdot3}+\frac{1}{3\cdot5}+...+\frac{1}{x\left(x+2\right)}\)
\(=\frac{1}{2}\cdot\left(1-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+...+\frac{1}{x}-\frac{1}{x+2}\right)\)
\(=\frac{1}{2}\cdot\left(1-\frac{1}{x+2}\right)\)
\(=\frac{1}{2}\cdot\frac{x+1}{x+2}\)
\(=\frac{x+1}{2x+2}\)
Đề sai bn ơi : Sửa : \(\frac{\frac{7}{3}+\frac{7}{4}-\frac{7}{11}}{\frac{5}{3}+\frac{5}{4}-\frac{5}{11}}+\frac{5}{7}\)
\(=\frac{7\cdot\left(\frac{1}{3}+\frac{1}{4}-\frac{1}{11}\right)}{5\cdot\left(\frac{1}{3}+\frac{1}{4}-\frac{1}{11}\right)}+\frac{5}{7}\)
\(=\frac{7}{5}+\frac{5}{7}=\frac{49}{35}+\frac{25}{35}=\frac{74}{35}\)
