81 : ……. x 3 = 27
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![](https://rs.olm.vn/images/avt/0.png?1311)
![](https://rs.olm.vn/images/avt/0.png?1311)
ta có :
\(\frac{x+1}{3}+\frac{x+1}{9}+\frac{x+1}{27}+\frac{x+1}{81}=\left(x+1\right)\times\left(\frac{1}{3}+\frac{1}{9}+\frac{1}{27}+\frac{1}{81}\right)\)
\(=\left(x+1\right)\times\frac{40}{81}=\frac{56}{81}\text{ nên }x+1=\frac{56}{40}=\frac{7}{5}\)
vậy \(x=\frac{7}{5}-1=\frac{2}{5}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
`@` `\text {Ans}`
`\downarrow`
\(\left(-3\right)^x\div81=-27\)
`=>`\(\left(-3\right)^x=-27\cdot81\)
`=>`\(\left(-3\right)^x=\left(-3\right)^3\cdot3^4\)
`=>`\(\left(-3\right)^x=\left(-3\right)^7\)
`=> x=7`
Vậy, `x=7.`
`@` `\text {Kaizuu lv u}`
(-3)x . 81 = -27
<=> (-3)x . (-3)4 = (-3)3
<=> (-3)x+4 = (-3)3
=> x+4=3
<=>x= -1
![](https://rs.olm.vn/images/avt/0.png?1311)
Vũ Hồng Linh bạn check lại bài đầu dùm =_="
\(\left[-\frac{1}{3}\right]^3\cdot x=\frac{1}{81}\)
\(\Leftrightarrow x=\frac{1}{81}:\left[-\frac{1}{3}\right]^3\)
\(\Leftrightarrow x=\frac{1}{81}:\left[-\frac{1}{27}\right]\)
\(\Leftrightarrow x=\frac{1}{81}\cdot(-27)=-\frac{1}{3}\)
\(\left[x-\frac{1}{2}\right]^3=\frac{1}{27}\)
\(\Leftrightarrow\left[x-\frac{1}{2}\right]^3=\left[\frac{1}{3}\right]^3\)
=> Làm nốt
Mấy bài kia cũng làm tương tự
(- \(\dfrac{1}{3}\))3.\(x\) = \(\dfrac{1}{81}\)
\(x=\dfrac{1}{81}\) : (- \(\dfrac{1}{3}\))3
\(x\) = - (\(\dfrac{1}{3}\))4 :(\(\dfrac{1}{3}\))3
\(x=-\dfrac{1}{3}\)
Vậy \(x=-\dfrac{1}{3}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
\(\frac{\left(-3\right)^x}{81}=-27\\ \Leftrightarrow\left(-3\right)^x=-27.81\\ \Leftrightarrow\left(-3\right)^x=-2187\\ \Leftrightarrow\left(-3\right)^x=\left(-3\right)^7\\ \Rightarrow x=7\)
![](https://rs.olm.vn/images/avt/0.png?1311)
-3^x/81 = - 27
-3^x = -27 . 81
-3^x = - 2187
\(\Rightarrow\)-3 ^ x = (-3 ) ^ 7
\(\Rightarrow\)x = 7
![](https://rs.olm.vn/images/avt/0.png?1311)
![](https://rs.olm.vn/images/avt/0.png?1311)
Ta có: \(\dfrac{\left(-3\right)^x}{81}=27\)
\(\Leftrightarrow\left(-3\right)^x=2187\)
hay \(x\in\varnothing\)
81 : ... \(\times\) 3 = 27
81: ... = 27 : 3
81: ... = 9
... = 81 : 9
... = 9