4.8.2 Greatest hits from previous weeks

4.8.2.1 How to add a vertical line to a plot

http://mathematica.stackexchange.com/questions/3561/how-to-add-a-vertical-line-to-a-plot

 
In the plot below I would like to add two vertical lines at $x = \frac{\pi}{15} \pm \frac{1}{20}$. 
How can I do that? 
 
f[x_] := (x^2 z)/((x^2 - y^2)^2 + 4 q^2 x^2) /. {y -> /15, z -> 1, q ->  ...
 

- asked by sjdh (33 votes), answered by Ajasja (49 votes)

4.8.2.2 Can Mathematica do symbolic linear algebra?

http://mathematica.stackexchange.com/questions/3242/can-mathematica-do-symbolic-linear-algebra

 
For instance, is there some way I can say "let A and B be arbitrary real $m\times n$ and $k\times m$ matrices, Simplify[Transpose[Transpose[A].Transpose[B]]]" and Mathematica would simplify it to B.A? 
...
 

- asked by nikie (42 votes), answered by helen (21 votes)