Question

The solution set of $\ln (5 - 7x) \leqslant 1$ is given by,
A. $\left[ {\dfrac{{5 - e}}{7},\dfrac{5}{7}} \right)$
B. $\left[ {\dfrac{{2 - e}}{3},\dfrac{2}{3}} \right)$
C. $( - 10,7)$
D. All real numbers

Answer 1

Hint: The set containing all the solutions of an equation is called the solution set for that equation. We can find the interval of $x$ by solving the inequality.
Use the property of logarithm;
$\Rightarrow$ ${e^{\ln (x)}} = x$
Logarithmic function and exponential function are the inverse function.

Complete step-by-step answer:
We are asked to find the solution set of $\ln (5 - 7x) \leqslant 1$.
Remove the logarithm by taking exponential both sides of the inequality $\ln (5 - 7x) \leqslant 1$.
$\Rightarrow$$(5 - 7x) \leqslant e$
Logarithmic function and exponential function are the inverse function.
$\Rightarrow$$5 - e \leqslant 7x$
Divide both the sides by $7$.
$\Rightarrow$\[\dfrac{{5 - e}}{7} \leqslant \dfrac{{7x}}{7}\]
$\Rightarrow$\[ \Rightarrow \dfrac{{5 - e}}{7} \leqslant x \ldots (1)\]
 All the solutions of $x$ greater than \[\dfrac{{5 - e}}{7}\].
Logarithmic function is defined for positive values.
$\Rightarrow$$5 - 7x > 0$
$\Rightarrow$$5 > 7x$
$\Rightarrow$$\dfrac{5}{7} > x \ldots (2)$
From the inequalities $(1)$ and $(2)$.
$\Rightarrow$$\dfrac{{5 - e}}{7} < x < \dfrac{5}{7}$
The solution set of $\ln (5 - 7x) \leqslant 1$ is $\left[ {\dfrac{{5 - e}}{7},\dfrac{5}{7}} \right)$.

Correct Answer: $\left[ {\dfrac{{5 - e}}{7},\dfrac{5}{7}} \right)$

Note:
The logarithmic function is defined for positive values.
Use the property of logarithm;
$\Rightarrow$${e^{\ln (x)}} = x$