@AzU I have your problem solved, but I'm not at a computer right now. I'll post it soon. But you really only need to do a few simple things to get there: 

1. Change Pel to P[el].
2. Get rid of the units, like I said before.
3. Add the numeric option to dsolve.

@AzU Ah, so T[a](t) is constant, not really a function of t. That makes things much easier. But what about P[el](t)? Certainly you'll get no response from dsolve without specifying it also.

Do you have an initial condition, for example T[ovn](0) = 400?

Although it is possible to include units directly in the problem specification, I think that you should avoid that for the time being, until you get a bit more experience with Maple. Just use compatible and consistent units and keep track of them mentally.

The Question area (this area) is the appropriate place for this. I deleted your duplicate of this from the Post area.

@Rouben Rostamian  Vote up. Excellent graphic presentation of an Answer. Nice abstraction through use of [...]. Nice use of vertical and horizontal shading to show direction of integration (this, plus some arrows, is how I teach double integrals).

@28th You're welcome. I'd guess that it's nearly certain that that solution is a transcendental number other than a rational multiple of Pi, and it has no symbolic representation other than "it is what it is". Proving things like that for specific numbers is notoriously difficult, yet it's easy to prove that almost every (a.e.) real number is transcendental. 

@Kitonum Order matters. He wants sublists, not subsets. But I don't know if contiguousness (contiguity?) matters.

@nm Please stop deleting Answers that have been responded to! Can't you see that it makes the whole thread hard to read?

@Kitonum There are two cases that are trickier to handle, both of which occur in the OP's set. Those are types Not(`+`) = 0 (i.e., a single term) and anything = Not(0). That's why my two-line procedure below took me 45 minutes to write.

@chinthak Please let me know whether my solution has resolved your problem.

Good find! I never would've noticed this if you hadn't pointed it out.

I can confirm that what you say also happens in Maple 2019.1 and that it didn't happen in Maple 2018, where the plot smoothly resizes and reshapes as you move the mouse.

You'll likely get Answers sooner if you either post your work as a worksheet or remove the plaintext output. If it's only plaintext input, I can copy-and-paste it; but if you include plaintext output (which is barely readable anyway) as in the above, it's a much bigger chore to execute your code.

@Christian Wolinski Your 5.4 example shows an ostensibly white sphere turned half peach and half light blue by light3. That doesn't seem very controllable, although you may appreciate the aesthetic. If there are only 4 light models, I think that they should all be white, especially since you can create custom colored lights.

@Stretto 

If by "amount of interpolation" you mean the degree of the piecewise polynomial pieces that make up the interpolating function, then that can be controlled by the degree option to SplineInterpolation. It defaults to 3, and this is widely regarded as the best choice for most purposes, such as for plotting a function. It's high enough that the curve has a continuous second derivative at the junctions of the pieces but not so high as to introduce spurious wiggles.

On the other hand, if you mean the number of interpolated points, then that can be controlled by the numpoints, adaptive, and sample options to plot (see ?plot,options).

@shkarah I solved Eqs 17-19 from the paper to get Eqs 20-22. Is that not what you wanted? The work in your worksheet seems to begin at an earlier place in the paper than the portion of it that you posted in your Question.

@Mac Dude 

In addition to their unevaluation action, the quotes also act like parentheses. But it is only this action as parentheses that is needed in your example:

eval(series(sin(g(x)), x), g= (x-> x^2+1));

The parentheses are needed because -> has a lower operator precedence than =.