@nm 

Thanks

It gives me the same result as yours code showed, so no need to contact Maple support.
Its old code and worked on earlier maple versions then?

I was too thinking on a returm, but this code looks strange without end proc and no ; for the statement

Also you defined input parameters too, so i don't know what the author of this code has in mind for this as a (confusing) example ?      

@acer 

Thanks

Because it was not working this procedure  and from this a long unsolved issue that  i never understood the use of a proc ( ) without a parameter definition

@vv 

Thanks

It works again

@nm 

Thanks

Seems to be now working with a new restart of Maple after two times of opening in Maple earlier with a error message like in my posted example  ?
It are old worksheets and get a overhaul in Maple and i thought that it was the cause of not plotting.

Its working now correct and see the integral curves  in the DEplot 

@Carl Love 
Thanks

Remarkable that all answers/results in Maple not automated can be checked
There is a human involved to sort this out with limitations also too as i understand it correct.

@acer 

Interesting, but al that do and don't  makes it not really easy.
Is it not possible in a "next level" of programming to simplify this ? 
In general ask myself can Maple be programmed to a higher level of user-friendlyness ?

Its a Achimedean Solid this regular tetrahedron ?
For someone who wants to learn the geom3d  package better  its a useful example 

See also https://www.mapleprimes.com/questions/230162-Calculating-Area-Of-Shadow-#answer271424

ode:=diff(f(x),x)=f(x)^(-1);
dsolve(ode);
                                   d          1  
                           ode := --- f(x) = ----
                                   dx        f(x)
                                (1/2)                     (1/2)
              f(x) = (2 x + _C1)     , f(x) = -(2 x + _C1)     

@janhardo 

@Carl Love 

Thanks

Well , i don't see it , thats why i could not figured out by meself

@nm 

Thanks

You are right for this type of ODE its solved by integrating and rather easy to solve. 

@vv 

Thanks

Yes, indeed its not function with multiplicative inverse., but a inverse function see 

https://en.wikipedia.org/wiki/Inverse_function

@nm 

Thanks

Just annother last question :how do you get the blue answers between the gray boxes ?  

@acer 

Thanks

Yes, i tried that first with the <pre> tags( no <quote> tags) ( made in past some websites , so tags were familiar) 
But get not the text between the two tags.
There were two  <P>  lines and the blocks get separated 
Tried several times and gave up too soon.

@nm 

DEtools['odeadvisor']( y(x) = 1*ln(cos(diff(y(x), x)))+(diff(y(x), x))*tan(diff(y(x), x)), y(x), 'help' );
                          [_dAlembert]
----------------------------------
DEtools['odeadvisor']( y(x) = ln(cos(diff(y(x), x)))+(diff(y(x), x))*tan(diff(y(x), x)), y(x), 'help' );
                          [_dAlembert]

For x= 1 you get a dAlembert ODE

@Carl Love 

Thanks

Now i can make a gray box  ( distracted by the two A's )