Unable to find elements in modal popup using selenium
What is a NullPointerException, and how do I fix it?
403 error after upgrading to apache2.4 After upgrading from Ubuntu 13.04 server, the www pages won't display themselves.     Error 403 Forbidden You don't have permission to access / on this server.   I've changed access control saves from apache2.2 to apache2.4, but that didn't work. What else could I do? Android Min SDK Version vs. Target SDK Version I noticed that sometimes my code gets highlighted in different colors when rendered.   What is syntax highlighting? How does it work? Why isn't my code being highlighted correctly? How do I report a bug or request a new language? How do I use syntax highlighting? What languages are currently available on Stack Exchange? Option to contact moderators Due to some "recent issues", I think the following is necessary:  Users need a way to contact moderators through the StackOverflow website.  This is especially important for suspensions, where there is no way to appeal or discuss the suspension from within SO itself.  Basically, there would be some kind of a box on the user's page that allows them to mention something to the moderators and some way to read the replies. (Or they could just go to the user's email.) How to allow to run su instantly after I added the user to the appropriate group Can a self-complementary graph on 100 vertices have precisely one vertex of degree 50? One confusion over conditional expectation Suppose for two random variables $X_1$ and $X_2$ $\sigma(X_1) = \sigma(X_2)$. Why $E[Y | X_1] = E[Y | X_2]$ a.e. ?  the set where $X_1= X_1(\omega)$ can be different from the set $X_2= X_2(\omega) $. How can then conditional expectation conditioned on the event can be equal ? Archimedes' principle for two liquid layers Problem: I have a cylindrical vessel of height $H$ and radius $R$. There are two liquid layers in the vessel. The first has density $D_1$ and height $h_1$, the second has density $D_2$ and height $h_2$. The second liquid is floating on the first liquid (thus $D_2 &lt; D_1$) and they are both entirely within the vessel ($h_1+h_2 \le H$). Now I have a cube of density $D$ and edge length $n$. The cube fits into the vessel ($n \le \sqrt2R$) but it might not be completely inside it ($n&gt;H$ is possible). I place it into the vessel such that one side is horizontal (the cube is not lopsided). Some liquid might overflow as a result. How do I use Archimedes' principle to calculate the topmost liquid level after placing the cube? I can ignore damping and other physical effects. EDIT: Note that the only values I know are $H, R, D_1, h_1, D_2, h_2, n$ and their abovementioned constraints.  My workings: I tried using the formula directly, where $\text{weight of cube}=\text{weight of displaced liquid}$. I first get $Dn^3=D_2n^2s_2$ where $s_2$ is the height submerged in the second liquid. If $s_2 \le h_2$ then the cube is only submerged in the second liquid. So the answer is $\min(H, h_1+h_2+{n^2s_2\over\pi R^2})$, accounting for overflows. This case is simple. But if $s_2&gt;h_2$ then the cube can be submerged in the first liquid too. I thought of subtracting the weight of the second liquid displaced to obtain the height submerged in first liquid, but realize that I don't know that. The weight displaced depends on the final liquid height, which can be affected by overflows, whether the cube fits totally within the two liquids, etc. In fact, it can be the case that after the liquid level rises, the cube now displaces more of the second liquid, causing it to not be submerged in the first (is that even possible?) It seems very messy and I have no idea how to start. :( Does anyone have a nice solution to this problem? Density of $\mathbb{Q}^n$ in $\mathbb{R}^n$ lstlisting: how to set code color and frame color I've been googling for some time now, and can't find the manual for the TikZ library calc. All I see is people using it. Where can I find the manual or some info?  Is it the same as the LaTeX package calc? The latter doesn't seem to mention TikZ in the manual. An application of derivatives problem- to show that $f(x+2)-f(x)>2$ for $f(x)=x\cos(1/x)$ I have this function $$f(x) = x\cos(1/x)$$ defined for all $x \geq 1$.      How do I prove that    $$f(x+2)-f(x)&gt;2$$    for all $x\geq 1$?   I found $$f'(x)= \cos(1/x) + \frac{\sin(1/x)}{x}$$ and $$f''(x)=-\frac{\cos(1/x)}{x^3}.$$ How do i use this information? Map "Super+ArrowDown" to minimize a window while the window is focused & the window is not in fullscreen I would like to add a conditional key mapping to Ubuntu 19.10.   Normally when a window is in full screen mode and you press super+arrowDown it enters window mode (or whatever it is called).   When a window now is in "window mode" I would like to be able to minimise it (and only then) by using super+arrowDown on it.  Is it possible to create such a shortcut without affecting the mapping of super+arrowDown on full screen windows? UnboundLocalError: local variable 'url_request' referenced before assignment UV coordinates on a deformed plane How to keep Heading together with text? I use a very custom style (not mine) and very frequently I get a heading  on the bottom of a page and the next paragraph on the top of the next page. Sometimes this paragraph may be a list or similar. The same is true for listings and the listing description.   Is there a general way how I can adjust the "badness" of these events? Or can I adjust when a section heading would be on the last, say, 3cm of a page it should go to the next page? Or can I say, if a *listing description" is on the top 3cm of a page pull 5 more listing lines from the last page onto this page?   The style I use does not use standard structure tags and environments. But lets call them section, subsection, listing and I will try to migrate them to my lingo. I'm well aware of the combinatorial variant of the proof, i.e. noting that each formula is a different representation for the number of subsets of a set of $n$ elements. I'm curious if there's a series of algebraic manipulations that can lead from $\sum\limits_{i=0}^n \binom{n}{i}$ to $2^n$. Series $\frac{x^{3n}}{(3n)!} $ find sum using differentiation $\mbox{rank}(A)=1$ implies $\det(A+I)=\mbox{trace}(A)+1$ This problem comes from exercise from my lecturer  Let $A$ be a square matrix such that $\mbox{rank}(A)=1$. Prove that $\det(A+I)=\mbox{trace}(A)+1$.  ($I$ is the identy matrix)  I usually haven't any idea in solving linear algebra problem. I'd like to find the files in the current directory that contain the text "chrome".  $ find . -exec grep chrome find: missing argument to `-exec'   What am I doing wrong?