@SHIVAS Thank you. I just put a link to this Question in that other thread.

@Suryakanth Here is a link to the nearly identical Question that I was sure that I'd seen: https://www.mapleprimes.com/questions/236809-Error-In-Ode-Plot-

@ Norma, have you tried the annotation option as I showed in my Answer? Although my code there is necessarily quite esoteric due to needing to use an essentially different computer language, MathML, for parts of it, I believe that it does exactly what you asked for: It gives the y-values for all curves in different colors in one "frame" for any given t value that your cursor is on (the cursor needs to be on any of the curves).

@C_R Left click on the plot to open its context menu, go to Probe Info, then check None.

@dharr In that case, I vote up. Thanks for clarifying the worksheet.

@dharr It's a good Answer, but one detail needs to be addressed: series has option system as well as option remember, which means that you can't rely on a remembered result staying remembered---they get cleared by garbage collection. 

@jrive You asked:

•  What do you mean by nontrivial solution?

Keep in mind that I'd misread that your intention was to solve for DC, T, and tau. In that case, I would consider 
DC = ton/(ton + toff) to be a trivial and undesirable solution (although technically correct), and I strove to avoid it.

@Gabriel Barcellos In that case, I don't understand why you called it an "Incorrect answer".

@jrive The confusion was my fault. I simply misread "in terms of" as "for". 

It would be clearer, at least for me, if this were stated at the bottom of the Question.

@acer Thanks. I hadn't tried NextZero because I assumed that it couldn't work on something that can't be symbolically differentiated. But it's the winner among the methods shown.

All three methods get a 3X time boost by using option remember in BC2.

Although it didn't seem to give any trouble in this case, one might need to be wary of the singularity at a = -1 in some other situations.

@Gabriel Barcellos What was your result from eval(A1, exp= cos)? I get 5, which is correct because the cos terms sum to 0, as you said.

I can't display the worksheet inline, so I'll copy in the plots:

@acer Okay, I can get all 4 values of a from fsolve without any plot-based seed values, thus making the implitcitplot irrelevant. My procedure SubDivide searches all real intervals (starting with -infinity..infinity), splitting each interval when it contains a root, and discarding an interval when fsolve comes back with nothing. This extra fsolveing (especially the ones that return empty) adds nearly 2 seconds. That can be  reduced to about 1.5 seconds by using option remember on procedure BC2.

SubDivide:= proc(P, {maxsols::posint:= 99})
local Q:= DEQueue(-infinity..infinity), sols:= 0, intv, r;
    {while sols < maxsols do
        intv:= pop_back(Q);
        r:= fsolve(P, intv, 'avoid'= ({op}(intv) minus {infinity, -infinity}));
        if r::numeric then sols++; Q,= lhs(intv)..r, r..rhs(intv); r fi
    until empty(Q)}
end proc
:
Digits:= 10: #precision for fsolve
a_sols:= SubDivide(BC2);
    a_sols := {-3.535939019, -2.002235920, -0.7734241018, 0.1155584192}

Those are the same values as before. Thus, the plotting procedes exactly as before.

The Maple 2023 help ?fsolve,details mentions a method= subdivide option. I tried to use it, but it seemed to be completely ignored! So, I implemented my own SubDivide.

@Carl Love MaplePrimes incorrectly formatted the GUI output (including plots) from my worksheet above. I have no idea why. You'll need to download it and view it directly in Maple.

@C_R The OP means GUI "equation/table" labels such as what one can get with Ctrl-L or, from the menus, Insert => Label.