"Don't tell this your math teacher" - Why?
Is $\frac{\textrm{d}y}{\textrm{d}x}$ not a ratio?
I am trying to find out how to approach a relief type project. I have a background image and a flat plane mesh to sculpt on, but I actually don´t see any changes looking straight on top. When I tilt the perspective a bit the image disappears.  Is there a way to make the image appear at all perspectives? How to sort a character vector where elements contain letters and numbers in R? Why am I getting authentication errors for packages from an Ubuntu repository? I'm experimenting with VPNs, so I wanted to install pptpd.  john@desktop:~$ sudo apt-get install pptpd [sudo] password for john:  &lt;snip&gt; The following extra packages will be installed:   bcrelay The following NEW packages will be installed:   bcrelay pptpd 0 upgraded, 2 newly installed, 0 to remove and 11 not upgraded. Need to get 90.5 kB of archives. After this operation, 442 kB of additional disk space will be used. Do you want to continue [Y/n]? y   And everything runs fine until I get down to here:  WARNING: The following packages cannot be authenticated!   bcrelay pptpd Install these packages without verification [y/N]? n E: Some packages could not be authenticated   No authentication?  I'm no security expert, but I'm fairly sure that's a bad thing.  So I .  john@desktop:~$ apt-cache policy pptpd pptpd:   Installed: (none)   Candidate: 1.3.4-3ubuntu1   Version table:      1.3.4-3ubuntu1 0         500 http://us.archive.ubuntu.com/ubuntu/ natty/main i386 Packages john@desktop:~$ apt-cache policy bcrelay bcrelay:   Installed: (none)   Candidate: 1.3.4-3ubuntu1   Version table:      1.3.4-3ubuntu1 0         500 http://us.archive.ubuntu.com/ubuntu/ natty/main i386 Packages   They're coming from Ubuntu.com and not being properly authenticated?  What's going on here? What is Ubuntu's status on the Meltdown and Spectre vulnerabilities? Any questions relating to status updates, or asking if anything is going to be patched for these vulnerabilities should be closed as duplicates of this question.    are in the news right now and sound pretty severe. I don't see any security updates from Ubuntu that cover these vulnerabilities.  What is Ubuntu doing about these vulnerabilities, and what should Ubuntu users be doing?  These are CVE-2017-5753, CVE-2017-5715 and CVE-2017-5754. Read Gif Images using WebView android Change owner of internal hard drive partition from root to user I have a number of drives internal and external. Some are partitioned. Most mount automatically at boot (all did at one point).    One partition is owned by root and therefore does not mount automatically. I know how to mount it as root.   I do not know how to take ownership of that partition as user X. I have tried changing permissions using the gui in Nautilus as root. They are not accepted and immediately revert.  The partition is NTFS. The other parition on that drive is owned by me as user and mounts automatically The age of the universe Many times I have read statements like, "the age of the universe is 14 billion years" . For example this wikipedia page .   Now, my question is, which observers' are these time intervals? According to whom 14 billion years? How to check iOS version? Why don't we include $\rho g h$ in a hydraulic jack below? Consider a hydraulic jack with massless pistons as follows.    The famous equation for this system is   $$ \frac{F_1}{A_1}=\frac{F_2}{A_2} $$  My question is why isn't the equation as follows?  $$ \frac{F_1}{A_1}=\frac{F_2}{A_2} + \rho g h $$  It is based on  my understanding that any points in the same horizontal line have the same pressure.  Could you spot my misconception? Listing layers from .mxd file using ArcPy for ArcGIS Pro? Rings, ideals and units-proof Let $R$ be a ring and let $J$ be an ideal of $R$. Assume $J$ contains   a unit of $R$. Prove $J=R$, rigorously.   Again I feel here as if the statement is intuitive so I am not able to make it rigorous. If you could show all steps in the proof that would be nice. Get total duration of video files in a directory I have a list of .ts files:   out1.ts ... out749.ts   out8159.ts  out8818.ts   How can I get the total duration (running time) of all these files? Is the humanoid appearance of most alien species in the Star Trek universe ever canonically addressed? The vast majority of intelligent species in the Star Trek universe are bipedal, humanoid creatures.  From the lot I've seen of TOS, TNG, DS9, Voyager, Enterprise, and the movies, it seems that this coincidence goes largely unquestioned.  Metaknowledge grants that it is much easier to have human actors portray human-like beings, and so this was probably done due to budgetary/technological limitations.  However, I'm wondering if an in-universe reason for this was ever given or explored? Is quantum entanglement affected by time dilation? Is quantum entanglement affected by time dilation? Let's say one of the entangled pair is accelerated to very high speed. When both the entangled particles are observed at the same time, will they have the opposite spin? How to show also the name of the parent counter with cleveref? Consider the following piece of code:  \documentclass{book} \usepackage{cleveref} \usepackage{lipsum}  \begin{document}   \chapter{Chapter One}   \section{Section One.One}   \label{mysection}   \lipsum    \section{Section One.Two}   Please ignore \cref{mysection}.    \chapter{Chapter Two}   \section{Section Two.One}    As you may have noticed, \cref{mysection} is useless. \end{document}   By default, in both uses of \cref I get Section 1.1 as expected.   Is it possible to change the way the reference is shown in such a way that the second use of \cref above will print Section 1 of Chapter 1? i.e., naming the parent counter as well.   However, this should be done only outside the own chapter, so the first use of \cref in the above example should print Section 1.  This comes from a use case of a normative text where the document is divided in "articles" and "commas", and it is good to have references like "Comma 1 of Article 2" or "Article 2, Comma 1". Does the series $\sum \limits _{n=0}^{\infty} \cos(n\pi)$ converge or diverge? Does the series $\sum \limits _{n=0}^{\infty} \cos(n\pi)$ converge or diverge?   On substituting values I get alternate $1$ and $-1$.    So taking sum of infinite GP, I get $\dfrac 1 2$. So it looks like it converges. Is there a comma needed before quotation? Boy and girl paradox is driving me crazy How to obtain optimal hyperparameters after nested cross validation? How can I permanently enable cheating?