# Finding value of (y) of logarithmic equation given (x)

I have an logarithmic equation $$\left[ r=a\,e^{b\,\theta} \right]$$

And I plot it to visualise it (see plot below). I can tell by the plot when (t=0), x1=0, y=1 (point AA) but how can I find out numerically what (y2) point BB will be when x2=.5 (the red dot on the plot (point BB)) by using an equation.

Eq Solve for a $$\left[ a={{r}\over{e^{b\,\theta}}} \right]$$

Eq to Solve for b $$\left[ b={{\log \left({{r}\over{a}}\right)}\over{\log e\,\theta }} \right]$$

Eq to Solve for theta $$\left[ \theta={{\log \left({{r}\over{a}}\right)}\over{b\,\log e }} \right]$$

I tried solving the equations using maxima but it came back with a large list of logs instead of a one numerical value.

kill(all);
r:.5; a:1; b:-5.7; theta:theta; solve(a*e^(b*theta)=r,theta);
tex(''%);


I'm still at a loss as to how to find (y2) at point BB when (x2=.5) (y2=?)

Ps: I'll be using octave 3.8.1 to calculate these values which is like matlab but I'm just trying to get the equations worked out correctly.

• How do $y_2$ and $x_2$ relate to the variables in your first equation? I suggest rewriting this in terms of one set of variables (or at least explaining what they're for). – horchler Aug 17 '15 at 14:00
• @RickT You need another (known) data point or something similar to evaluate the coordinates of BB. – callculus Aug 17 '15 at 14:19
• @horchler y2 and x2 are just the x and y values for point BB x2 and t are the same thing. I'll fix the plot and update – Rick T Aug 17 '15 at 14:25
• It looks like $x$ is just $\theta$ and $r$ is $y$. Your question would be a lot clearer if you picked one set or the other. – horchler Aug 17 '15 at 14:28
• @RickT Now I understand your problem. You just had to insert the values in your equation $\theta=\frac{\frac{log(0.5)}{1}}{-5.7\cdot log (e)}=\frac{log(0.5)}{-5.7\cdot 1}\approx 0.121605$ – callculus Aug 17 '15 at 15:41

Say you're trying to solve for $\theta$ (i.e., $x_2$) in $r = a e^{b \theta}$ and are unable to find an analytical solution. You can use numerical root-finding. In Matlab and Octave fzero (documentation) solves for roots/zeros of simple univariate functions. The first step is to re-write your equation as $0 = a e^{b \theta}-r$. Then, with th0 as an initial guess, you can solve for a root via:

a = 1;
b = -5.7;
r = 0.5;
th0 = 1;
f = @(th)a*exp(b*th)-r;
[th_sol,fval,exitflag] = fzero(f,th0)


This returns 0.121604768519289 for th_sol. In this case, this is equivalent to the analytical solution $\theta = \text{log}(r/a)/b$.

Of course if you're solving for $y_2$ (i.e., $r$), you don't need to do anything special. Just evaluate:

a = 1;
b = -5.7;
th = 0.5;
r = a*exp(b*th)