Question

Tìm x:

a) \(\left(5x-1\right)^6=729\) b) \(\dfrac{8}{25}=\dfrac{2^x}{5^{x-1}}\) c) \(\left(\dfrac{1}{16}\right)^x=\left(\dfrac{1}{2}\right)^{10}\) d) \(9^x:3^x=3\)

Answer 1

Lời giải:

a) \((5x-1)^6=729=3^6=(-3)^6\)

\(\Rightarrow \left[\begin{matrix} 5x-1=3\\ 5x-1=-3\end{matrix}\right.\Rightarrow \left[\begin{matrix} x=\frac{4}{5}\\ x=\frac{-2}{5}\end{matrix}\right.\)

b)

\(\frac{8}{25}=\frac{2^x}{5^{x-1}}=\frac{2^x}{5^x:5}=5.(\frac{2}{5})^x\)

\(\Rightarrow \frac{8}{125}=(\frac{2}{5})^x\)

\(\Rightarrow (\frac{2}{5})^3=(\frac{2}{5})^x\Rightarrow x=3\)

c)

\((\frac{1}{16})^x=(\frac{1}{2})^{10}\)

\(\Rightarrow (\frac{1}{2^4})^x=(\frac{1}{2})^{10}\)

\(\Rightarrow (\frac{1}{2})^{4x}=(\frac{1}{2})^{10}\Rightarrow 4x=10\Rightarrow x=\frac{5}{2}\)

d)

\(9^{x}:3^x=3\Rightarrow (\frac{9}{3})^x=3\)

\(\Rightarrow 3^x=3^1\Rightarrow x=1\)