\(\left(\frac{1}{11}-\frac{1}{13}+\frac{1}{13}-...+\frac{1}{19}-\frac{1}{21}\right).462-\left[2,04:\left(x+1,05\right)\right]:0,12=19\)

\(\left(\frac{1}{11}-\frac{1}{21}\right).462-\left[2,04:\left(x+1,05\right)\right]:0,12=19\)

\(\frac{10}{231}.462-\left[2,04:\left(x+1,05\right)\right]:0,12=19\)

\(20-\left[2,04:\left(x+1,05\right)\right]:0,12=19\)

\(\left[2,04:\left(x+1,05\right)\right]:0,12=20-19=1\)

\(2,04:\left(x+1,05\right)=0,12\)

\(x+1,05=2,04:0,12\)

x+1,05=17

x=17-1,05=15,95

mk chỉ làm được câu a thôi : 

a) gọi A= \(\frac{2}{11.13}+\frac{2}{13.15}+......+\frac{2}{19.21}\)

A= \(\frac{1}{11}-\frac{1}{13}+\frac{1}{13}-\frac{1}{15}+......+\frac{1}{19}-\frac{1}{21}\)

     \(\frac{1}{11}-\frac{1}{21}\)

       \(\frac{21}{231}-\frac{11}{231}\)= \(\frac{10}{231}\)

\(\Rightarrow\frac{10}{231}.462-[2,04:\left(x+1,05\right)]:0,12=19\)

20,08 - \([2,04:\left(x+1,05\right)]:0,12=19\)

\(.........\)

\(\left(\frac{2}{11.13}+\frac{2}{13.15}+...+\frac{2}{19.21}\right).462-\left[2,04:\left(x+1,05\right):0,12\right]=19\)

\(\Leftrightarrow\left(\frac{1}{11}-\frac{1}{13}+\frac{1}{13}-\frac{1}{15}+...+\frac{1}{19}-\frac{1}{21}\right).462-\left[2,04:\left(x+1,05\right):0,12\right]=19\)

\(\Leftrightarrow\left(\frac{1}{11}-\frac{1}{21}\right).462-\left[2,04:\left(x+1,05\right):0,12\right]=19\)

\(\Leftrightarrow\frac{10}{231}.462-\left[2,04:\left(x+1,05\right):0,12\right]=19\)

\(\Leftrightarrow20-\left[2,04:\left(x+1,05\right):0,12\right]=19\)

\(\Leftrightarrow2,04:\left(x+1,05\right):0,12=1\)

\(\Leftrightarrow2,05:\left(x+1,05\right)=0,12\)

\(\Leftrightarrow x+1,05=\frac{205}{12}\)

\(\Leftrightarrow x=\frac{205}{12}-1,05\)

\(\Leftrightarrow x=\frac{481}{30}\)

Sao mk câu hỏi của bạn dùng mà tốn giấy vậy, tiết kiệm chút đi

\(\left(\frac{2}{11.13}+\frac{2}{13.15}+\frac{2}{15.17}+\frac{2}{17.19}+\frac{2}{19.21}\right)\) ) . 462 - x = 19

\(\left(\frac{1}{11}-\frac{1}{13}+\frac{1}{13}-\frac{1}{15}+\frac{1}{15}-\frac{1}{17}+\frac{1}{17}-\frac{1}{19}+\frac{1}{19}-\frac{1}{21}\right)\) . 462 - x  = 19

\(\left(\frac{1}{11}-\frac{1}{21}\right)\) . 462 - x = 19 

... Chúc bạn học tốt !

\(\left(\frac{2}{11\cdot13}+\frac{2}{13\cdot15}+\frac{2}{15\cdot17}+\frac{2}{17\cdot19}+\frac{2}{19\cdot21}\right)\)\(\cdot462-x=19\)

\(\left(\frac{1}{11\cdot13}+\frac{1}{13\cdot15}+\frac{1}{15\cdot17}+\frac{1}{17\cdot19}+\frac{1}{19\cdot21}\right)\)\(\cdot462-x=19\)

\(\left(\frac{1}{11}-\frac{1}{13}+\frac{1}{13}-\frac{1}{15}+\frac{1}{15}-\frac{1}{17}+\frac{1}{17}-\frac{1}{19}+\frac{1}{19}-\frac{1}{21}\right)\)\(\cdot462-x=19\)

\(\left(\frac{1}{11}-\frac{1}{21}\right)\cdot462-x=19\)

\(\frac{10}{231}\cdot462-x=19\)

\(20-x=19\)

\(x=20-19\)

\(x=1\)

cảm ơn mọi người