@one_man  Thank you for your suggestion! But I want the fractions to be displayed in 2D mode in beautiful style. Such output is important to me in one application.  See my solution in this thread.

@Carl Love  For example, should be  

rather than

or

@beidouxing   The quote "So  x-y  is a solution for the  eqn1=0 , so the eqn1 can be divided by the  x-y".  This is true only for rational expressions.

For example,  eval(1 - exp(x-y), x=y) = 0 , but you can not divide  1 - exp(x-y)  by  x-y

You should submit a complete version of your problem, ie, all the equations of the system and etc,  rather than only one equation.

@Markiyan Hirnyk   Yes, you're right. A higher degree is needed.

V := <t, piecewise(t >= 0, t^5, 0), piecewise(t >= 0, 0, -t^5)>;

Student[VectorCalculus][Torsion](V, t);

Student[VectorCalculus][SpaceCurve](V, t = -1 .. 1);

@Markiyan Hirnyk  For example, if you need to find the derivative at a point, then you can find  separately left and right derivatives and make sure they match.

@Markiyan Hirnyk 

V1:=<t, t^3, 0>;

V2:=<t, 0, -t^3>;

Student[VectorCalculus][Torsion](V1, t);

Student[VectorCalculus][Torsion](V2, t);

eval(Student[VectorCalculus][Curvature](V1, t), t=0);

Obviously  piecewise(t>=0, V1, t<0, V2)  is not a plane curve.

@Markiyan Hirnyk  If you are able to supplement my answer, no one is stopping you from doing so.

@taro  Compare two examples:

restart;

assign(a, c);

a := d;

a, c, d;

and

restart;

a := c;

assign(a, d);

a, c, d;

@taro  Quote "what is assing(a,c)? Does it mean both a:=c and c:=a?"

No. This means that in the future, writing  a  we mean  с  and  if after this we write assign(a,d)  then it means that с is d  and  a  is  d .

@Carl Love  Of course with your example there are no problems. But the initial question is general in nature and we do not know the original expression, in which  the substitution is done. My example just shows that even at this stage  problems may occur.

@Alejandro Jakubi   Of course I mean this obvious result, but Maple does not make it.

@Preben Alsholm  and  @Carl Love  In symbolic  sum  or  Sum  the commands  algsubs  or  eval  don't work:

S:=unapply(Sum(w[k], k=1..n), n) assuming n::posint:

algsubs(S(n)=Sw, S(n+1));

eval(S(n+1), S(n)=Sw);

@Konstantin@  In fact, your joke has a profound meaning. As you know, any truth is concrete. You're not going to turn on the computer, for example, to count 25 * 15, and do this in your mind or on a calculator.

@Carl Love   print screen -> Paint -> cutting -> saving -> inserting (by fat green arrow)

@Markiyan Hirnyk  Quote "You demonstrate the double moral standards, not regarding my work".

I think your moralizing is inappropriate and unacceptable.