Given the inequality:
$$\frac{64}{2^{x + 3}} \leq \left(-1\right) 7 + \left(\frac{1}{8}\right)^{x}$$
To solve this inequality, we must first solve the corresponding equation:
$$\frac{64}{2^{x + 3}} = \left(-1\right) 7 + \left(\frac{1}{8}\right)^{x}$$
Solve:
Given the equation:
$$\frac{64}{2^{x + 3}} = \left(-1\right) 7 + \left(\frac{1}{8}\right)^{x}$$
or
$$\left(7 - \left(\frac{1}{8}\right)^{x}\right) + \frac{64}{2^{x + 3}} = 0$$
Do replacement
$$v = \left(\frac{1}{8}\right)^{x}$$
we get
$$64 \cdot 2^{- x - 3} + 7 - 8^{- x} = 0$$
or
$$64 \cdot 2^{- x - 3} + 7 - 8^{- x} = 0$$
do backward replacement
$$\left(\frac{1}{8}\right)^{x} = v$$
or
$$x = - \frac{\log{\left(v \right)}}{\log{\left(8 \right)}}$$
$$x_{1} = \frac{- \log{\left(14 \right)} + \log{\left(-1 + \sqrt{29} \right)}}{\log{\left(2 \right)}}$$
$$x_{2} = \frac{\log{\left(\frac{1}{14} + \frac{\sqrt{29}}{14} \right)} + i \pi}{\log{\left(2 \right)}}$$
$$x_{3} = \frac{i \pi}{\log{\left(2 \right)}}$$
Exclude the complex solutions:
$$x_{1} = \frac{- \log{\left(14 \right)} + \log{\left(-1 + \sqrt{29} \right)}}{\log{\left(2 \right)}}$$
This roots
$$x_{1} = \frac{- \log{\left(14 \right)} + \log{\left(-1 + \sqrt{29} \right)}}{\log{\left(2 \right)}}$$
is the points with change the sign of the inequality expression.
First define with the sign to the leftmost point:
$$x_{0} \leq x_{1}$$
For example, let's take the point
$$x_{0} = x_{1} - \frac{1}{10}$$
=
$$\frac{- \log{\left(14 \right)} + \log{\left(-1 + \sqrt{29} \right)}}{\log{\left(2 \right)}} - \frac{1}{10}$$
=
$$\frac{- \log{\left(14 \right)} + \log{\left(-1 + \sqrt{29} \right)}}{\log{\left(2 \right)}} - \frac{1}{10}$$
substitute to the expression
$$\frac{64}{2^{x + 3}} \leq \left(-1\right) 7 + \left(\frac{1}{8}\right)^{x}$$
$$\frac{64}{2^{\left(\frac{- \log{\left(14 \right)} + \log{\left(-1 + \sqrt{29} \right)}}{\log{\left(2 \right)}} - \frac{1}{10}\right) + 3}} \leq \left(-1\right) 7 + \left(\frac{1}{8}\right)^{\frac{- \log{\left(14 \right)} + \log{\left(-1 + \sqrt{29} \right)}}{\log{\left(2 \right)}} - \frac{1}{10}}$$
/ ____\ / ____\
29 -log(14) + log\-1 + \/ 29 / 1 -log(14) + log\-1 + \/ 29 /
- -- - --------------------------- <= -- - ---------------------------
10 log(2) 10 log(2)
64*2 -7 + 8
the solution of our inequality is:
$$x \leq \frac{- \log{\left(14 \right)} + \log{\left(-1 + \sqrt{29} \right)}}{\log{\left(2 \right)}}$$
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