How do you make the graph for y=ln(1+x/(ln(1-x)))?

2 Answers
Feb 19, 2017

It looks to me like Socratic graphing cannot plot it for some reason.
This is the graph:

enter image source here

And my calculator can plot it also:

enter image source here

See Socratic graph and explanation.

Explanation:

To make y real, 1-x > 0, giving x < 1 and

1+x/ln(1-x)>0, giving

x/ln(1-x) > -1, with x < 1.

So, if x in (0, 1), ln(1-x)<0, and so,

x<-ln(1-x)<=1. This gives

x>=1-e^(-1)=0.63212, nearly.

Also, the upper terminal of the graph is (1_1, 0_-) and the lower

end is (1-1/e, -1).

Now, x in [1-1/e, 1), the the graph is illustrative of this aspect of the

function. I realize that the lower end is lower than (1-1/e, -1).

I hope that , with the interest already shown, Steve could study my

answer and give his comments.

graph{x-(e^y-1)ln(1-x)=0 [-2.5, 2.5, -1.25, 1.25]}

@a-s-adikesavan

I've embedded my comments in the answer rather than as a comment due to all the maths:

For the domain of the function:

Condition 1

1-x>0 so that ln(1-x) in RR => color(red)(x<1)
This is consistent with your answer

Condition 2
I get a different result for the 2nd condition; as follows'

1+x/ln(1-x) > 0 so that ln(1+x/ln(1-x)) in RR

As we are solving an inequality we need to be careful with the sign of the denominator ln(1-x) and change the inequality direction as appropriate.

Using the fact that for any A in RR:

ln A \ \ \ { (>0), (=0), (<0) :} \ \ iff {: (A>1), (A=1), (A<1) :}

Lets examine the three cases:

Case 1: \ \ color(blue)(ln(1-x)>0)

ln(1-x)>0 =>1-x>1=>x<0
With this requirement, we have:
1+x/ln(1-x) > 0
:. ln(1-x)+x>0 (as ln(1-x)>0)
And this has no real solutions:

Case 2: \ \ color(blue)(ln(1-x)=0)

ln(1-x)=0 =>1-x=1=>x=0
This case would lead to x/ln(1-x) being of an indeterminate form 0/0, so lets use L'Hôpital's rule to find the limit:
lim_(x rarr 0)(1+x/ln(1-x)) = 1+lim_(x rarr 0)(x/ln(1-x))
" "= 1+lim_(x rarr 0)(1/(1/(1-x)(-1)))
" "= 1-1
" "= 0
and as we require 1+x/ln(1-x)>0 we can eliminate this.

Case 3: \ \ color(blue)(ln(1-x)<0)

ln(1-x)<0 =>1-x<1=>x>0
With this requirement, we have:
1+x/ln(1-x) > 0
:. ln(1-x)+x<0 (as ln(1-x)<0)
Using WFA and graphs this has the solution color(red)(x>0)

Combining all the results gives the domain of y as x in (0,1), which is inconsistent with your analysis.

Thanks to some help from @jimh we have

lim_(x rarr 0)ln(1+x/ln(1-x)) rarr -oo

But it does so extremely slowly due to the effect of the log function, and so the range is y in (-oo,0)

Due to the fact that very rapidly we can be dealing with very small numbers with many decimal places, the computer accuracy is a issue when displaying the graph of the function.

Eg A~~1xx10^-10 => ln(A) ~~ -23 so the graph appear to "tail off" in the region -25 lt y lt -15 but this is an illusion and for a sufficiently small xthen y decreases without bound.

Other than looking at derivatives I'm not sure there is much more to add