$a=(float) 640.81; $b=(float) 636.85; $c=(float) 3.96;  $d= $a-$b-$c; echo $d;   Why  am getting -7.7271522513911E-14 this value? It should be 0 right.
Consider the following code:   0.1 + 0.2 == 0.3  -&gt;  false   0.1 + 0.2         -&gt;  0.30000000000000004   Why do these inaccuracies happen?
Is it possible to combine processing power of 2 computers? What is the physical significance of the off-diagonal moment of inertia matrix elements? The tensor of moment of inertia contains six off-diagonal matrix elements, which vanish if we choose a reference frame aligned with the principal axes of the rotating rigid body; the angular momentum vector is then parallel to the angular velocity. But while considering the general case, what are the off-diagonal moment of inertia matrix elements? That is, do they have any physical significance as [say] the components of a vector? Or is it merely a mathematical construction with no definite physical meaning (which seems rather wrong to me)?  A similar thread exists  but they are more interested in the principal axes of the body. It also says:     The physical significance of the off-diagonal components is that   you're using a coordinate system not aligned with the principal   directions of the object. They tell us nothing interesting about the   object itself.   Is that all or is there more to it, perhaps related to properties of tensors in general? I have the following configuration, but I have not able to find any documentation on how to set a maximum backup files on date rolling style.  I know that you can do this with size rolling style by using the maxSizeRollBackups.  &lt;appender name="AppLogFileAppender" type="log4net.Appender.RollingFileAppender"&gt;     &lt;file value="mylog.log" /&gt;     &lt;appendToFile value="true" /&gt;     &lt;lockingModel type="log4net.Appender.FileAppender+MinimalLock" /&gt;     &lt;rollingStyle value="Date" /&gt;     &lt;datePattern value=".yyMMdd.'log'" /&gt;     &lt;layout type="log4net.Layout.PatternLayout"&gt;       &lt;conversionPattern value="%d %-5p %c - %m%n"  /&gt;     &lt;/layout&gt; &lt;/appender&gt; How do we control web page caching, across all browsers? Our investigations have shown us that not all browsers respect the HTTP cache directives in a uniform manner.  For security reasons we do not want certain pages in our application to be cached, ever, by the web browser. This must work for at least the following browsers:   Internet Explorer 6+ Firefox 1.5+ Safari 3+ Opera 9+ Chrome   Our requirement came from a security test. After logging out from our website you could press the back button and view cached pages. Prove that $x-1$ divides $x^n-1$ In algebra &amp; polynomials, how do we prove that $$x-1 \mid x^n -1?$$ The cube of any number not a multiple of $7$, will equal one more or one less than a multiple of $7$ Yeah so I'm kind of stuck on this problem, and I have two questions.   Is there a way to define a number mathematically so that it cannot be a multiple of $7$? I know $7k+1,\ 7k+2,\ 7k+3,\ \cdots$, but that will take ages to prove for each case.  Is this a proof? $$(7k + a)^3 \equiv (0\cdot k + a)^3 \equiv a^3 \bmod 7$$    Thank you. How do I use the Natural Earth SQLite DB with QGIS? I just downloaded the Natural Earth data in SQLite format from . I had assumed that this would be a SpatiaLite database, but it appears not to be! QGIS is failing to recognize it as a spatial database. OGR supposedly supports reading geometries even when stored in a plain SQLite database, but maybe QGIS doesn't use OGR for SpatiaLite?  The SQLite database does have geometry_columns and spatial_ref_sys tables. Is there some way to convert it to a full SpatiaLite database? Unknown Column In Where Clause Solve congruence system $\ x\equiv m_i-1 \pmod{m_i}\,$ for $\,i = 1,\ldots, k$ I'm running Apache2 in a docker container, and want to write nothing to the disk, writing logs to stdout and stderr. I've seen a few different ways to do this (,  ) but these seem like hacks. Is there no way to do this by default?   To be clear, I do not want to tail the log, since that will result in things being written to the disk in the container. How can I prevent URLDownloadToFile from retrieving from the cache? I am using URLDownloadToFile to retrieve a file from a website. Subsequent calls return the original file rather than an updated version.  I assume it is retrieving a cached version. Windows Spotlight Source and Save locations? I've only played one placement match so far and I won it because my opponents were less than stellar.  I'm afraid that if I get similar placement matches in the future and end up winning them all I might get put into a league beyond my skill.  How many different leagues are there?  Once placed in a league, is there any way to move to a different one?  How? Using proof by contradiction vs proof of the contrapositive I'm new to both JavaScript and . In YUI library examples, you can find many uses of this construct:  (function() {     var Dom = YAHOO.util.Dom,     Event = YAHOO.util.Event,     layout = null,         ... })();   I think the last couple of parentheses are to execute the function just after the declaration.  ... But what about the previous set of parentheses surrounding the function declaration?  I think it is a matter of scope; that's to hide inside variables to outside functions and possibly global objects. Is it? More generally, what are the mechanics of those parentheses? While I am familiar with Descartes and the oft-quoted idea, I think therefore I am, I am not familiar with any philosophy that actively sought to rationalize that anybody else exists.  Are there philosophers who analyzed this question? Or is it taken for granted that it is (im)possible for anybody to validate their own existence to somebody else? Highlighting the selected tab using jQueryUI Tabs I have a header page with links to all the visualforce page as tabs.I need to highlight the tab on select of it.      &lt;apex:page title="{!$Label.site.site_login}" showHeader="false" sidebar="false"       standardStylesheets="false"&gt; &lt;apex:includeScript value="{!URLFOR($Resource.jQuery, '/js/jquery-1.4.2.min.js')}"  /&gt;   &lt;apex:includeScript value="{!URLFOR($Resource.jQuery, '/js/jquery-ui-1.8.6.custom.min.js')}"  /&gt; &lt;apex:stylesheet value="{!URLFOR($Resource.jQuery, '/css/ui-lightness/jquery-ui-1.8.6.custom.css')}"  /&gt;          &lt;html lang="en"&gt;  &lt;head&gt;   &lt;script src="http://ajax.googleapis.com/ajax/libs/jqueryui/1.8/jquery-ui.min.js"&gt;  $(function()  {   $( "#tabs" ).tabs();   });  &lt;/script&gt;   &lt;style&gt;   #tabs .ui-tabs-active    { background: red; }  &lt;/style&gt;     &lt;/head&gt;      &lt;body&gt;         &lt;nav class="navbar navbar-static-top navbar-inverse" role="navigation"&gt;             &lt;div class="container"&gt;                 &lt;div class="navbar-header"&gt;                     &lt;button type="button" class="navbar-toggle" data-toggle="collapse" data-target=".navbar-ex1-collapse"&gt;                         &lt;span class="icon-bar"&gt;&lt;/span&gt;&lt;span class="icon-bar"&gt;&lt;/span&gt;                         &lt;span class="icon-bar"&gt;&lt;/span&gt;                      &lt;/button&gt;                     &lt;a class="navbar-brand" style="line-height: 0px; padding: 10px 15px;" href="index.php.html"&gt;                         &lt;img height="30px" src="{!URLFOR($Resource.Resources, 'img/logo.png ')}" /&gt;                     &lt;/a&gt;                 &lt;/div&gt;                 &lt;div class="collapse navbar-collapse navbar-ex1-collapse" id="tabs"&gt;                     &lt;ul class="nav navbar-nav"&gt;                         &lt;li id="Home"&gt;                             &lt;apex:outputLink value="{!$Page.Home}"&gt;Home&lt;/apex:outputLink&gt;                         &lt;/li&gt;                         &lt;li id="chatter"&gt;                             &lt;apex:outputLink value="{!$Page.Chatter}"&gt;Chatter&lt;/apex:outputLink&gt;                         &lt;/li&gt;                         &lt;li id="page3"&gt;                             &lt;apex:outputLink value="{!$Page.page3}"&gt;Room&lt;/apex:outputLink&gt;                         &lt;/li&gt;                         &lt;li id="page4"&gt;                             &lt;apex:outputLink value="{!$Page.page4}"&gt;Page4&lt;/apex:outputLink&gt;                         &lt;/li&gt;                     &lt;/ul&gt;                 &lt;/div&gt;             &lt;/div&gt;         &lt;/nav&gt;     &lt;/body&gt;      &lt;/html&gt; &lt;/apex:page&gt; While modeling an Audi R8 I have the following problem. The thing I want to create is seen in this image:    I want to create a sharp crease (outlined in red) which starts around the first exhaust and then goes to the second.  Is there an efficient way to do this (I also use the Subdivision Surface Modifier for this model). I have a website that sells music artists posters. Each artists has a page containing a small bio, and a list of posters we are selling representing the artist.  Currently our HTML looks like that  &lt;h1&gt;{Artist name}&lt;/h1&gt; bio &lt;h2&gt;Posters&lt;/h2&gt; &lt;ul&gt;   &lt;li class="poster"&gt;...&lt;/li&gt;   ... &lt;/ul&gt;   We are redesigning this page, and we have a new template that we really like. Problem is that in this new version the &lt;h2&gt;Posters&lt;/h2&gt; does not appear anywhere on the page.  As you might expect, a significant part of our traffic comes from people typing something like : "{Artist name} poster" on Google.  I fear that, by removing the word "poster" from the page, we will reduce our chances of getting these people to come to the site, since Google might down rank us.  I see several options here :   Give Google a slightly altered page that contains the H2 tags. But I think this falls into the "Black Hat SEO" category. And I don't want that. Put the heading. Hide it with CSS. Everyone wins, including accessibility tools which can determine the structure of the page using the "invisible" heading. But I do not know if this is considered a good practice. Forget about the heading, and find other ways to tell Google that we really are about posters. (via keywords, but do they really have the same "weight" as an H2?) Adapt the design to our SEO need, and find a way to put that heading nicely.   Are there webmasters out there that have been facing the same issue, how did you resolve it? I am going to install Ubuntu to dual boot with Windows 8 soon, but one of my friends warned me that partitioning my hard drive will slow my computer down. I run on a 500GB HDD and 8GB of ram if that helps at all. Is that true if I do it?