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`#3107.101107`
\(\left(3^x-81\right)\left(x^2+1\right)=0\)
\(\Rightarrow\left[{}\begin{matrix}3^x-81=0\\x^2+1=0\end{matrix}\right.\\ \Rightarrow\left[{}\begin{matrix}3^x=81\\x^2=-1\left(\text{vô lý}\right)\end{matrix}\right.\\ \Rightarrow\left[{}\begin{matrix}3^x=3^3\\x\in\varnothing\end{matrix}\right.\\ \Rightarrow\left[{}\begin{matrix}x=3\\x\in\varnothing\end{matrix}\right.\)
Vậy, `x = 3.`
![](https://rs.olm.vn/images/avt/0.png?1311)
\(1,\Leftrightarrow\left[{}\begin{matrix}7x+2=0\\x-1=0\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}x=-\dfrac{2}{7}\\x=1\end{matrix}\right.\\ 2,\Leftrightarrow\left[{}\begin{matrix}1-3x=\dfrac{1}{9}\\3x-1=\dfrac{1}{9}\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}3x=\dfrac{8}{9}\\3x=\dfrac{10}{9}\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}x=\dfrac{8}{27}\\x=\dfrac{10}{27}\end{matrix}\right.\)
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[2x + 1]^ 3 = 125
[2x + 1]^ 3 = 5^3
=> 2x + 1 = 5
=> 2x = 4
=> x = 2
[3x - 2]^4 = -81
Ta có 1 số bất kì mũ chẵn sẽ bằng số dương
=> x k thỏa
[4x - 8]^ 3 = 64
=> [4x - 8]^3 = 4^3
=> 4x - 8 = 4
=> 4x = 12
=> x = 3
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\(\Rightarrow\left(3x+4\right)^3=-125=\left(-5\right)^3\\ \Rightarrow3x+4=-5\\ \Rightarrow3x=-9\\ \Rightarrow x=-3\)
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