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Why does solve() produces an error? TypeError: The first argument must be a symbolic expression or a list of symbolic expressions.

#!/usr/bin/env python
# coding: utf-8

# In[2]:


var('Ex Ey Ez')


# In[3]:


E=vector([Ex,Ey,Ez])


# In[4]:


var('Px Py Pz')


# In[5]:


P=vector([Px,Py,Pz])


# In[6]:


var('LAx LAy LAz LBx LBy LBz')


# In[7]:


LA=vector([LAx,LAy,LAz]); LB=vector([LBx,LBy,LBz])


# In[8]:


var('k QAx QAy')


# In[9]:


QA=QAx*LA+QAy*LB


# In[20]:


solve(E-P==k*(E-QA), [k, QAx, QAy])


# In[24]:


solve((E-P).cross_product(E-QA)==0, [QAx, QAy])

Let E, P, QA be points of a 3D space, let a*QAx+b*QAy+c*QAz==d ensures that QA is in a certain plane. Let |E P| and |E QA| be the same line. Why did my attempt to find QA found zero solutions?

#!/usr/bin/env python
# coding: utf-8

# In[1]:


var('a b c d')


# In[2]:


var('Ex Ey Ez')


# In[3]:


E=vector([Ex,Ey,Ez])


# In[4]:


var('Px Py Pz')


# In[5]:


P=vector([Px,Py,Pz])


# In[6]:


var('k QAx QAy QAz')


# In[7]:


QA=vector([QAx,QAy,QAz])


# In[14]:


solve([E-P==k*(E-QA), a*QAx+b*QAy+c*QAz==d], [k, QAx, QAy, QAz])

I need to write a short math text draft containing some Sage calculations. (I am a Sage novice.)

What software (or software combinations) you may suggest to do this?

Is it possible to do with TeXmacs? If yes, does it require to install any TeXmacs addons? I tried wiki.sagemath.org/TeXmacs but it does not work (does not appear in the list of installed plugins) with my version of TeXmacs.

I prefer interactive software rather than LaTeX for this task.