Gọi 3 cạnh tam giác là a,b,c(m;a,b,c>0)

Áp dụng tc dtsbn:

\(\dfrac{a}{3}=\dfrac{b}{4}=\dfrac{c}{6}=\dfrac{a+b+c}{3+4+6}=\dfrac{42}{13}\\ \Leftrightarrow\left\{{}\begin{matrix}a=\dfrac{42}{13}\cdot3=\dfrac{126}{13}\left(m\right)\\b=\dfrac{42}{13}\cdot4=\dfrac{168}{13}\left(m\right)\\c=\dfrac{42}{13}\cdot6=\dfrac{252}{13}\left(m\right)\end{matrix}\right.\)

Vậy ...

\(\Leftrightarrow16x=64\Leftrightarrow x=4\)

\(=5x^2-3x-x^3+x^2+x^3-6x^2-10+3x=0\)

\(ĐK:x\ne\pm2\\ 1,E=\dfrac{x^2+x^2-3x+2-x-2}{\left(x-2\right)\left(x+2\right)}=\dfrac{2x\left(x-2\right)}{\left(x-2\right)\left(x+2\right)}=\dfrac{2x}{x+2}\\ 2,E=\dfrac{3}{4}\Leftrightarrow\dfrac{2x}{x+2}=\dfrac{3}{4}\Leftrightarrow3x+6=8x\Leftrightarrow x=\dfrac{6}{5}\left(tm\right)\\ 3,\left|x-4\right|=2\Leftrightarrow\left[{}\begin{matrix}x-4=2\\4-x=2\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}x=6\left(tm\right)\\x=2\left(ktm\right)\end{matrix}\right.\Leftrightarrow x=6\\ \Leftrightarrow E=\dfrac{6\cdot2}{6+2}=\dfrac{12}{8}=\dfrac{3}{2}\)

\(\dfrac{2x}{3}=\dfrac{3y}{4}=\dfrac{4z}{5}\Leftrightarrow\dfrac{x}{\dfrac{3}{2}}=\dfrac{y}{\dfrac{4}{3}}=\dfrac{z}{\dfrac{5}{4}}=\dfrac{3x-2y+4z}{3\cdot\dfrac{3}{2}-2\cdot\dfrac{4}{3}+4\cdot\dfrac{5}{4}}=\dfrac{-164}{\dfrac{41}{6}}=-24\\ \Leftrightarrow\left\{{}\begin{matrix}x=-24\cdot\dfrac{3}{2}=-36\\y=-24\cdot\dfrac{4}{3}=-32\\z=-24\cdot\dfrac{5}{4}=-30\end{matrix}\right.\)

\(\left(1\right)FeCl_2+2NaOH\to Fe\left(OH\right)_2\downarrow +2NaCl\\ \left(2\right)Fe\left(OH\right)_2\xrightarrow{t^0}FeO+H_2O\\ \left(3\right)FeO+C\to Fe+CO\uparrow \\ \left(4\right)2Fe+3Cl_2\xrightarrow[]{t^o}2FeCl_3\\ \left(5\right)FeCl_3+3NaOH\to Fe\left(OH\right)_3\downarrow +3NaCl\\ \left(6\right)2Fe\left(OH\right)_3\xrightarrow{t^o}Fe_2O_3+3H_2O\\ \left(7\right)Fe_2O_3+3CO\xrightarrow{t^o}2Fe+3CO_2\\ \left(8\right)Fe+2HCl\to FeCl_2+H_2\)

Đặt \(n_{O_2}=x\left(mol\right)\)

\(2Cu\left(NO_3\right)_2\xrightarrow[]{t^o}2CuO+4NO_2\uparrow+O_2\\ \Rightarrow n_{NO_2}=4a\\ \Rightarrow22,4.\left(4a+a\right)=5,6\\ \Rightarrow a=0,05\left(mol\right)\\ \Rightarrow n_{Cu\left(NO_3\right)_2}=2a=0,1\left(mol\right)\\ \Rightarrow m_{Cu\left(NO_3\right)_2}=0,1.188=18,8\left(g\right)\)

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