Chapter 3.3, Problem 84E

To determine

To find: the logarithmic equation that relates y to x .

Answer to Problem 84E

  $\ln y=\frac{2}{3}\ln x$

Explanation of Solution

Given:

The table:

x	1	2	3	4	5	6
y	1	0.630	0.481	0.397	0.342	0.303

Concept Used:

Slope of a line:

The slope of line passing through two point $\left(x_1,y_1\right)$ and $\left(x_2,y_2\right)$ is given by $m=\frac{y_2-y_1}{x_2-x_1}$ , $x_1\ne x_2$

Slope intercept form of equation of line is $y=mx+b$ , where m is the slope of the line and $\left(0,b\right)$ is the y -intercept of the line.

Construct a new table by taking the natural logarithms of the x and y values in the given table.

$\ln x$	0	0.693	1.099	1.386	1.609	1.792
$\ln y$	0	-0.462	-0.732	-0.924	-1.073	-1.194

Now plot each point $\left(\ln x,\ln y\right)$ to make sure they lie on a line.

  

Now, choose any two points to determine the slope of the line.

So, using the points $\left(0,0\right)$ and $\left(0.693,-0.462\right)$ , the slope of the line is:

  $\begin{array}{c}m=\frac{y{2}_{}-y_1}{x_2-x_1}\\ =\frac{-0.462-0}{0.693-0}\\ =\frac{2}{3}\end{array}$

Now, from the slope-intercept form of line, the equation of line is:

  $Y=mX+b$

Where $Y=\ln y$ and $x=\ln x$

From the above graph, it is clear that the y -intercept of line is 0. That is, $b=0$

Thus, the equation of line is:

  $\begin{array}{c}Y=mX\\ \ln y=\frac{2}{3}\ln x\end{array}$

Conclusion:

The logarithmic equation that relates y to x is $\ln y=\frac{2}{3}\ln x$ .