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

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

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

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

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

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

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

x+1,05=17

x=17-1,05=15,95

Tính cái ngoặc đầu trước rồi ráp lại sau nhé.

* \(\frac{2}{11.13}+\frac{2}{13.15}+...+\frac{2}{19.21}\)

\(=2\left(\frac{1}{11.13}+\frac{1}{13.15}+...+\frac{2}{19.21}\right)\)

\(=2\left(\frac{1}{11}-\frac{1}{21}\right)\)

\(=2.\frac{10}{231}\)

\(=\frac{20}{231}\)

Ta có: \(\frac{20}{231}.462-\left[0,04:\left(x+1,05\right)\right]:0,12=19\)

         \(\Leftrightarrow40-\left[0,04:\left(x+1,05\right)\right]:0,12=19\)

          \(\Leftrightarrow\left[0,04:\left(x+1,05\right)\right]:0,12=21\)

           \(\Leftrightarrow0,04:\left(x+1,05\right)=2,52\)

          \(\Leftrightarrow x+1,05=\frac{1}{63}\)

           \(\Leftrightarrow x=-\frac{1303}{1260}\)

PS: Có những chỗ ra kết quả luôn, không tính vì mất thời gian. Nhưng bạn vẫn hiểu được đó, yên tâm.

\(\left(\dfrac{2}{11\cdot13}+\dfrac{2}{13\cdot15}+...+\dfrac{2}{19\cdot21}\right)\cdot462-\left[0,04:\left(x+1,05\right)\right]:0,12=19\)\(\Leftrightarrow\dfrac{2}{2}\cdot\left(\dfrac{1}{11}-\dfrac{1}{13}+\dfrac{1}{13}-\dfrac{1}{15}+...+\dfrac{1}{19}-\dfrac{1}{21}\right)\cdot462-\left[\dfrac{1}{25}:\left(x+\dfrac{21}{20}\right)\right]:\dfrac{3}{25}=19\)\(\Leftrightarrow\left(\dfrac{1}{11}-\dfrac{1}{21}\right)\cdot462-\left[\dfrac{1}{25}:\left(x+\dfrac{21}{20}\right)\right]:\dfrac{3}{25}=19\)

\(\Leftrightarrow20-\left[\dfrac{1}{25}:\left(x+\dfrac{21}{20}\right)\right]:\dfrac{3}{25}=19\)

\(\Leftrightarrow\left[\dfrac{1}{25}:\left(x+\dfrac{21}{20}\right)\right]:\dfrac{3}{25}=1\)

\(\rightarrow\dfrac{1}{25}:\left(x+\dfrac{21}{20}\right)=\dfrac{3}{25}\)

\(\Leftrightarrow x+\dfrac{21}{20}=\dfrac{1}{3}\)

\(\Rightarrow x=\dfrac{1}{3}-\dfrac{21}{20}=-\dfrac{43}{60}\)

Vậy \(x=-\dfrac{43}{60}\)

cái này cồng kềnh nhưng ko khó - >ngại. Bn tính 2/11*13+.... trước bằng cách rút gọn sau đó nhân ra -> dễ

\(\Leftrightarrow\left(\frac{1}{11}-\frac{1}{13}+\frac{1}{13}-\frac{1}{15}+...+\frac{1}{19}-\frac{1}{21}\right).462-\frac{17}{x+1,05}=19.\)

\(\Leftrightarrow\left(\frac{1}{11}-\frac{1}{21}\right)462-\frac{17}{x+1,05}=19\)

\(\Leftrightarrow20-\frac{17}{x+1,05}=19\)

\(\Leftrightarrow1-\frac{17}{x+1,05}=0\)

\(\Leftrightarrow x+1,05=17\)

\(\Leftrightarrow x=15,95\)