3.44.2 Greatest hits from previous weeks

Greatest hits from previous weeks:

3.44.2.1 Matrix in Latex

https://tex.stackexchange.com/questions/204621/matrix-in-latex

I am new to Latex, and I have been trying to get the matrix of following form 
[x11 x12 x13 . . . . x1n 
x21 x22 x23 . . . . x2n 
. 
. 
. 
. 
xd1 xd2 xd3 . . . . xdn] 
...
 

- asked by Lakshmi Narayanan (66 votes), answered by skpblack (103 votes)

3.44.2.2 How can I split an equation over two (or more) lines

https://tex.stackexchange.com/questions/3782/how-can-i-split-an-equation-over-two-or-more-lines

I am having the following equation: 
\begin{equation} 
Q(\lambda,\hat{\lambda}) = -\frac{1}{2} P(O \mid \lambda ) \sum_s \sum_m \sum_t \gamma_m^{(s)} (t) \left( n \log(2 \pi ) + \log \left| C_m^{(s)}  ...
 

- asked by Peter Smit (171 votes), answered by Will Robertson (123 votes)