title: Maxima can't solve lineare equation with sum
body: <p>I'm trying to calculate the equation for linear regression with maxima but maxima can't solve the linear system:</p>
  
  <pre><code>Maxima 5.38.1 http://maxima.sourceforge.net
  using Lisp GNU Common Lisp (GCL) GCL 2.6.12
  Distributed under the GNU Public License. See the file COPYING.
  Dedicated to the memory of William Schelter.
  The function bug_report() provides bug reporting information.
  (%i1) e: sum((y[i] - (a*x[i]+b))^2, i, 1, n);
                               n
                              ====
                              \                    2
  (%o1)                        &gt;    (y  - a x  - b)
                              /       i      i
                              ====
                              i = 1
  (%i2) dea: diff(e, a, 1);
                                n
                               ====
                               \
  (%o2)                    - 2  &gt;    x  (y  - a x  - b)
                               /      i   i      i
                               ====
                               i = 1
  (%i3) deb: diff(e, b, 1);
                                  n
                                 ====
                                 \
  (%o3)                      - 2  &gt;    (y  - a x  - b)
                                 /       i      i
                                 ====
                                 i = 1
  (%i4) linsolve([dea, deb], [a, b]);
  (%o4)                                 []
  </code></pre>
  
  <p>Why is it empty? As far as I see maxima can't solve for variables in sums with some other variables.  How can I fix it? Do I have to use another function or do I have to reorder my equations?</p>