$x$ is a vector and $A$ is a matrix and I'm confused as to how to find the partial derivatives of $x^TAx$ with respect to $x_1$, $x_2$, etc.
For the Quadratic Form $X^TAX; X\in\mathbb{R}^n, A\in\mathbb{R}^{n \times n}$ (which simplifies to $\Sigma_{i=0}^n\Sigma_{j=0}^nA_{ij}x_ix_j$), I tried to take the derivative wrt. X ($\Delta_X X^TAX$) and ended up with the following:  The $k^{th}$ element of the derivative represented as  $\Delta_{X_k}X^TAX=[\Sigma_{i=1}^n(A_{ik}x_k+A_{ki})x_i] + A_{kk}x_k(1-x_k)$  Does this result look right? Is there an alternative form?  I'm trying to get to the $\mu_0$ of Gaussian Discriminant Analysis by maximizing the log likelihood and I need to take the derivative of a Quadratic form. Either the result I mentioned above is wrong (shouldn't be because I went over my arithmetic several times) or the form I arrived at above is not the terribly useful to my problem (because I'm unable to proceed).  I can give more details about the problem or the steps I put down to arrive at the above result, but I didn't want to clutter to start off. Please let me know if more details are necessary.  Any link to related material is also much appreciated.
I've just answered .  After accepting the answer, the OP then changed his question to another problem - which I know I shouldn't have answered (I should have requested as new question) but I updated my answer anyway.  The OP has now edited his question to remove the essence of either the original or new problem, and instead has a statement saying that the problem has been fixed.  What is the correct thing to do here?  Roll the question back so it's in its original state? Roll it back to the "new problem" state? Leave it and let him continue to get downvoted and/or closed by people who didn't see the original question? Create a local texmf tree in MiKTeX In TeX Live there are TEXMFLOCAL for admin and TEXMFHOME for user pre-defined, but in MiKTeX there’s by default no local texmf tree installed, though it is useful, cf. . Therefore my question:  How do I create a local texmf tree in MiKTeX? I'm using LyX and a custom class from  which is based on the  class.  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