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\(\left(\frac{1}{11}-\frac{1}{13}+\frac{1}{13}-...+\frac{1}{19}-\frac{1}{21}\right).462-\left[2,04:\left(x+1,05\right)\right]:0,12=19\)
\(\left(\frac{1}{11}-\frac{1}{21}\right).462-\left[2,04:\left(x+1,05\right)\right]:0,12=19\)
\(\frac{10}{231}.462-\left[2,04:\left(x+1,05\right)\right]:0,12=19\)
\(20-\left[2,04:\left(x+1,05\right)\right]:0,12=19\)
\(\left[2,04:\left(x+1,05\right)\right]:0,12=20-19=1\)
\(2,04:\left(x+1,05\right)=0,12\)
\(x+1,05=2,04:0,12\)
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
\(\Leftrightarrow\left(\frac{1}{11}-\frac{1}{13}+\frac{1}{13}-\frac{1}{15}+...+\frac{1}{19}-\frac{1}{21}\right).462-\frac{17}{x+1,05}=19.\)
\(\Leftrightarrow\left(\frac{1}{11}-\frac{1}{21}\right)462-\frac{17}{x+1,05}=19\)
\(\Leftrightarrow20-\frac{17}{x+1,05}=19\)
\(\Leftrightarrow1-\frac{17}{x+1,05}=0\)
\(\Leftrightarrow x+1,05=17\)
\(\Leftrightarrow x=15,95\)