# Question about SDMB's "No homework questions" policy re math questions specifically **URL:** \ **Category:** The BBQ Pit\ **Created:** [October 17, 2004, 6:04pm UTC](https://boards.straightdope.com/t/question-about-sdmbs-no-homework-questions-policy-re-math-questions-specifically/269695 "2004-10-17T18:04:08Z")\ **Posts on this page:** 20\ **Page:** 2
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [October 17, 2004, 8:58pm UTC](https://boards.straightdope.com/t/question-about-sdmbs-no-homework-questions-policy-re-math-questions-specifically/269695/21 "2004-10-17T20:58:14Z")
> [@astro](#): > > That’s a pretty sloppy policy IMO, and quite frankly it forces busy mods to work out the nature of a question’s nuances… This is little different from many other types of problematic posts. The mods frequently have to make decisions on whether a post is hate speech, “being a jerk”, violative of US law, an unnecessary game thread, excessively argumentative, threatening, trolling and so on. One more thing isn’t going to kill them, especially something they’be been dealing with all along. Merely codifying a Board policy on this matter is not going to make any additional work for them. ---
**Author:** ![iampunha](https://avatars.discourse-cdn.com/v4/letter/i/74df32/32.png) [@iampunha](https://boards.straightdope.com/u/iampunha)\ **Post date:** [October 17, 2004, 9:05pm UTC](https://boards.straightdope.com/t/question-about-sdmbs-no-homework-questions-policy-re-math-questions-specifically/269695/22 "2004-10-17T21:05:32Z")
> [@Johnny L.A.](#): > > I’ve never seen this formula. Is it a “standard” formula, like the Pythagorean Theorem? That is, if you have a quadratic equation, is the solution _always_ > > ```auto > > -b± sqrt(b[sup]2[/sup] - 4ac) > x = ------------------ > 2ac > > ``` > > ? Your denominator should be 2A, not 2AC. Otherwise, as far as I know, yes with the following caveat: If your discriminant (that’s the part that goes b^2, or b[sup]2[/sup], minus 4AC, or simply the part under the sqrt sign) is negative, your results are not going to be real numbers. Real here is, from what I know, not exactly the same as the common meaning given to real. A real number is defined [here](http://en.wikipedia.org/wiki/Real_number). If your discriminant is not real, it will be imaginary (and thus in the form of a+b_i_, where a is a real number, b is the coefficient of _i_ and _i_ is the square root of -1), which is also defined (and linked) on that same page. **ultrafilter** might know of a more succinct definition, but for our purposes I think that one works well enough. But regardless, what does this mean? It basically means that if you’re using a thinking machine (calc, computer, whatever) to figure the roots of a problem, it’s going to give you an error if you solutions to a quadratic equation are imaginary and it’s operating in real number mode. If you (can) change it to imaginary number mode (a+b_i_ on a TI-X programmable calculator, where X could be several different numbers from I think the 80s up), you’ll get values such as this, for example: 4+5_i_ 4-5_i_ That’s the display you’ll get, at least, from the quadratic programs I’ve written. It’s the simplest way I know to display such a beast. ---
**Author:** ![Johnny\_L.A](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johnny_l.a/32/1084_2.png) [@Johnny\_L.A](https://boards.straightdope.com/u/Johnny_L.A)\ **Post date:** [October 17, 2004, 9:18pm UTC](https://boards.straightdope.com/t/question-about-sdmbs-no-homework-questions-policy-re-math-questions-specifically/269695/23 "2004-10-17T21:18:14Z")
```auto -b± sqrt(b2 - 4ac) x = ------------------ 2ac ``` Yes, the c was a typo. That’s a very useful formula! I wish I had known it when I was in school. It would have made things _much_ easier. I wonder why we were never taught it? Incidentally, I posted earlier that 4x[sup]2[/sup] - 15 + 9 didn’t look right. This is because I wasn’t looking hard enough. I figured out that it factors to (4x-3)(x-3), but dividing by (x-3) gave me .75 when I _knew_ the “answer” was 3. ---
**Author:** ![amarinth](https://avatars.discourse-cdn.com/v4/letter/a/3be4f8/32.png) [@amarinth](https://boards.straightdope.com/u/amarinth)\ **Post date:** [October 17, 2004, 9:19pm UTC](https://boards.straightdope.com/t/question-about-sdmbs-no-homework-questions-policy-re-math-questions-specifically/269695/24 "2004-10-17T21:19:33Z")
> [@Johnny L.A.](#): > > I’ve never seen this formula. Is it a “standard” formula, like the Pythagorean Theorem? That is, if you have a quadratic equation, is the solution _always_ > > ```auto > > -b± sqrt(b[sup]2[/sup] - 4ac) > x = ------------------ > 2 **a** > > ``` > > ? Yes, it’s a standard formula. Yes, it is always true. (And your teachers absolutely should have shown it to you…several times. Many things in pre-calc are not going to make sense at all, if you don’t have that.) Just to show work, if you have an equation: ax[sup]2[/sup]+bx+c=0 you use the “completing the squares” rules on it - so multiply by 4a to get 4a[sup]2[/sup]x[sup]2[/sup]+4abx+4ac=0 subtract 4ac from each side, and add b[sup]2[/sup] for: 4a[sup]2[/sup]x[sup]2[/sup]+4abx+ b[sup]2[/sup]= b[sup]2[/sup]-4ac factor the left half (2ax+b)[sup]2[/sup]=b[sup]2[/sup]-4ac take the square roots - 2ax+b=sqrt(b[sup]2[/sup]-4ac) and finish it to ```auto -b± sqrt(b[sup]2[/sup] - 4ac) x = ------------------ 2a ``` ---
**Author:** ![Johnny\_L.A](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johnny_l.a/32/1084_2.png) [@Johnny\_L.A](https://boards.straightdope.com/u/Johnny_L.A)\ **Post date:** [October 17, 2004, 9:36pm UTC](https://boards.straightdope.com/t/question-about-sdmbs-no-homework-questions-policy-re-math-questions-specifically/269695/25 "2004-10-17T21:36:16Z")
> [@](#): > > And your teachers absolutely should have shown it to you…several times. Now I feel cheated. ☹ Question about “completing the squares”: Do you always multiply by 4a and add b[sup]2[/sup]? I mean, is that “part of the rule”? > [@](#): > > Many things in pre-calc are not going to make sense at all, if you don’t have that. Trig and pre-calc were the same class, so they were probably abbreviated. I’d like to thank everyone for the assistance. ---
**Author:** ![ZebraShaSha](https://avatars.discourse-cdn.com/v4/letter/z/eada6e/32.png) [@ZebraShaSha](https://boards.straightdope.com/u/ZebraShaSha)\ **Post date:** [October 17, 2004, 9:52pm UTC](https://boards.straightdope.com/t/question-about-sdmbs-no-homework-questions-policy-re-math-questions-specifically/269695/26 "2004-10-17T21:52:10Z")
> [@Johnny L.A.](#): > > Now I feel cheated. ☹ > > Question about “completing the squares”: Do you always multiply by 4a and add b[sup]2[/sup]? I mean, is that “part of the rule”? > > Trig and pre-calc were the same class, so they were probably abbreviated. > > I’d like to thank everyone for the assistance. Uh… I do not know what he was talking about, myself. **Completing the square** ax^2 + bx = 0 Take b and divide by two, then square it: (b/2)^2 Add to both sides: x^2 + bx + (b/2)^2 = (b/2)^2 Which will equal: (x+(b/2)^2)^2 = (b/2)^2 Hopefully this helps! ---
**Author:** ![ZebraShaSha](https://avatars.discourse-cdn.com/v4/letter/z/eada6e/32.png) [@ZebraShaSha](https://boards.straightdope.com/u/ZebraShaSha)\ **Post date:** [October 17, 2004, 10:08pm UTC](https://boards.straightdope.com/t/question-about-sdmbs-no-homework-questions-policy-re-math-questions-specifically/269695/27 "2004-10-17T22:08:50Z")
Thinking about it, I should just derive the quadratic formula. ax^2 + bx + c = 0 Dividing by a: x^2 + (b/a)x + (c/a) = 0 Subtracting (c/a): x^2 + (b/a)x = -(c/a) Completing the squares: x^2 + (b/a)x + (b/2a)^2 = -(c/a) + (b/2a)^2 Which goes to: (x + (b/2a))^2 = -(c/a) + (b^2/4a) (x + (b/2a))^2 = (b^2 - 4ac)/(4a^2) Take the sqrt: x + (b/2a) = +/- sqrt(b^2-4ac)/(2a) Subtracting, we get: x = (-b/2a) + +/- sqrt(b^2-4ac)/(2a) Or: x = (-b +/- sqrt(b^2-4ac)) / 2a This was fun, I am with the OP. (sorry for the hijak, though) ---
**Author:** ![Thudlow\_Boink](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/thudlow_boink/32/320_2.png) [@Thudlow\_Boink](https://boards.straightdope.com/u/Thudlow_Boink)\ **Post date:** [October 18, 2004, 1:25am UTC](https://boards.straightdope.com/t/question-about-sdmbs-no-homework-questions-policy-re-math-questions-specifically/269695/28 "2004-10-18T01:25:58Z")
I’m guessing that part of the reason for the “homework rule” is to keep people from asking questions that absolutely no one cares about. If _you_ don’t care about it except insofar as it allows you to complete an assignment, and you don’t think anyone else would find it even remotely interesting, don’t clutter up the message board with it. I think math question are great. As long as they’re not just, “Here, solve these ten equations and don’t bother me with how you did it,” of course. ---
**Author:** ![ultrafilter](https://avatars.discourse-cdn.com/v4/letter/u/3d9bf3/32.png) [@ultrafilter](https://boards.straightdope.com/u/ultrafilter)\ **Post date:** [October 18, 2004, 1:53am UTC](https://boards.straightdope.com/t/question-about-sdmbs-no-homework-questions-policy-re-math-questions-specifically/269695/29 "2004-10-18T01:53:23Z")
> [@iampunha](#): > > If your discriminant (that’s the part that goes b^2, or b[sup]2[/sup], minus 4AC, or simply the part under the sqrt sign) is negative, your results are not going to be real numbers. Real here is, from what I know, not exactly the same as the common meaning given to real. A real number is defined [here](http://en.wikipedia.org/wiki/Real_number). If your discriminant is not real, it will be imaginary (and thus in the form of a+b_i_, where a is a real number, b is the coefficient of _i_ and _i_ is the square root of -1), which is also defined (and linked) on that same page. **ultrafilter** might know of a more succinct definition, but for our purposes I think that one works well enough. Yeah, there’s not much need to go into the specifics of what a real number is. That’s good enough. ---
**Author:** ![amarinth](https://avatars.discourse-cdn.com/v4/letter/a/3be4f8/32.png) [@amarinth](https://boards.straightdope.com/u/amarinth)\ **Post date:** [October 18, 2004, 3:13am UTC](https://boards.straightdope.com/t/question-about-sdmbs-no-homework-questions-policy-re-math-questions-specifically/269695/30 "2004-10-18T03:13:11Z")
> [@Johnny L.A.](#): > > Question about “completing the squares”: Do you always multiply by 4a and add b[sup]2[/sup]? I mean, is that “part of the rule”? No, I knew where I was going, so I multiplied to make the derivation easier. As far as completing squares, sometimes factoring is harder to see. But, every quadratic equation is equal to some linear equation squared plus a constant. So, instead solving for the quadratic, you figure out what the linear equation is, what the constant is, and then set the linear = to the positive and negative square roots of the constant… so in the homework example … original equation: 4x[sup]2[/sup]-15x+9=0 (4x[sup]2[/sup]-15x+225/16)-81/16=0 linear equation^2+constant: (2x-15/4)[sup]2[/sup]-81/16=0 (2x-15/4)[sup]2[/sup]=81/16 2x-15/4=±9/4 or x=3/4, x=3 ---
**Author:** ![Tikster](https://avatars.discourse-cdn.com/v4/letter/t/8edcca/32.png) [@Tikster](https://boards.straightdope.com/u/Tikster)\ **Post date:** [October 18, 2004, 6:35pm UTC](https://boards.straightdope.com/t/question-about-sdmbs-no-homework-questions-policy-re-math-questions-specifically/269695/31 "2004-10-18T18:35:52Z")
As a high school math teacher, I simply MUST butt in here. The proofs for the formula: x= (-b±sqrt(b^2-4ac))/2a That **amarinth** and **ZebraShaSha** present are both correct - which proof you find easiest is entirely up to you. And you ask if the multiplying by 4a and so firth is standard - the answer is yes. “a” denotes any given value in the ax^2+bx+c=0 equation. a could be 1 or 4, doesn’t matter which, the proof applies in all cases (which is why it’s more convenient to use the a, b and c in the first place). I hope this made sense, if not, can I use the excuse that I’m not used to explaining this in english? Tikster ---
**Author:** ![Thudlow\_Boink](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/thudlow_boink/32/320_2.png) [@Thudlow\_Boink](https://boards.straightdope.com/u/Thudlow_Boink)\ **Post date:** [October 18, 2004, 7:14pm UTC](https://boards.straightdope.com/t/question-about-sdmbs-no-homework-questions-policy-re-math-questions-specifically/269695/32 "2004-10-18T19:14:34Z")
> [@Tikster](#): > > yes. “a” denotes any given value in the ax^2+bx+c=0 equation. a could be 1 or 4, doesn’t matter which, the proof applies in all cases If you read something like this, and feel compelled to point out that, no, if doesn’t apply in _quite all_ cases (_a_ can’t be 0)— you just might be a math geek! (Of course, if a=0, it’s not really a quadratic equation, just a linear one, and you don’t _need_ any fancy-schmancy quadratic formula to solve it.) ---
**Author:** ![Tikster](https://avatars.discourse-cdn.com/v4/letter/t/8edcca/32.png) [@Tikster](https://boards.straightdope.com/u/Tikster)\ **Post date:** [October 18, 2004, 7:19pm UTC](https://boards.straightdope.com/t/question-about-sdmbs-no-homework-questions-policy-re-math-questions-specifically/269695/33 "2004-10-18T19:19:17Z")
You are correct of course 🙂 Good thing you’re not my pupil or I’d owe you cake! Tikster ---
**Author:** ![5que](https://avatars.discourse-cdn.com/v4/letter/5/3bc359/32.png) [@5que](https://boards.straightdope.com/u/5que)\ **Post date:** [October 18, 2004, 7:37pm UTC](https://boards.straightdope.com/t/question-about-sdmbs-no-homework-questions-policy-re-math-questions-specifically/269695/34 "2004-10-18T19:37:25Z")
My Brain Hurts! ::Smashing bricks against sides of head:: ---
**Author:** ![MysteryFellow63427](https://avatars.discourse-cdn.com/v4/letter/m/a88e4f/32.png) [@MysteryFellow63427](https://boards.straightdope.com/u/MysteryFellow63427)\ **Post date:** [October 18, 2004, 7:49pm UTC](https://boards.straightdope.com/t/question-about-sdmbs-no-homework-questions-policy-re-math-questions-specifically/269695/35 "2004-10-18T19:49:26Z")
My thinking is close to **astro** ’s on this topic. I’d also like to add that no hijack is more painful than a math hijack. Kee-rist. ---
**Author:** ![Yeticus\_Rex](https://avatars.discourse-cdn.com/v4/letter/y/a698b9/32.png) [@Yeticus\_Rex](https://boards.straightdope.com/u/Yeticus_Rex)\ **Post date:** [October 18, 2004, 7:56pm UTC](https://boards.straightdope.com/t/question-about-sdmbs-no-homework-questions-policy-re-math-questions-specifically/269695/36 "2004-10-18T19:56:14Z")
I love seeing math problems on the board here; it helps me remember what I’ve lost over the years. I think that the members here are not just “homework copiers”, but smart folks who just need a little bump here and there to solve something that’s stumped them for an inordinate amount of time. Also, I think there a quite a few people here who love to help those in need. Kinda builds up the health of the board when there is this type of exchange going on. I can see helping any of the charter members, but it can also attract new members when they see this type of exchange. Just so it doesn’t become a “finish my homework” session, I think the OP should always state what they have done so far with the problem as well as the problem itself. **Johnny LA** ’s problem can be solved so many ways, and I think the beauty here is that there were other posters who told how to do it those different ways. **Johnny** could pick the most comfortable way to approach the problem, or he could also study the other methods presented to increase his proficiency in dealing with future math problems that will inevitably come his way. That is what makes this board soooooo kewl. ---
**Author:** ![dropzone](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dropzone/32/7515_2.png) [@dropzone](https://boards.straightdope.com/u/dropzone)\ **Post date:** [October 18, 2004, 9:08pm UTC](https://boards.straightdope.com/t/question-about-sdmbs-no-homework-questions-policy-re-math-questions-specifically/269695/37 "2004-10-18T21:08:50Z")
> [@Q.E.D.](#): > > …and if you are up front about stating that you’re asking for help with homework. Really? I’d think with some of the quick-draw artists we have that would be the express train to Closedthreadville, with a threat of banning if you try it again. ---
**Author:** ![pretend\_my\_name\_is\_witty](https://avatars.discourse-cdn.com/v4/letter/p/b9e5f3/32.png) [@pretend\_my\_name\_is\_witty](https://boards.straightdope.com/u/pretend_my_name_is_witty)\ **Post date:** [October 18, 2004, 10:13pm UTC](https://boards.straightdope.com/t/question-about-sdmbs-no-homework-questions-policy-re-math-questions-specifically/269695/38 "2004-10-18T22:13:03Z")
> [@Thudlow Boink](#): > > I’m guessing that part of the reason for the “homework rule” is to keep people from asking questions that absolutely no one cares about. If _you_ don’t care about it except insofar as it allows you to complete an assignment, and you don’t think anyone else would find it even remotely interesting, don’t clutter up the message board with it. > > I think math question are great. As long as they’re not just, “Here, solve these ten equations and don’t bother me with how you did it,” of course. I was the guy who opened the homework thread that was closed. I asked for help in understanding _how_ to answer the question, not simply for the answer! So if you like math questions…and the kind people who replied to my thread did…then how are homework questions _“questions that absoultely no one cares about”_? Sorry to clutter up your message board. I’ll clear threads and posts with you next time! ---
**Author:** ![pretend\_my\_name\_is\_witty](https://avatars.discourse-cdn.com/v4/letter/p/b9e5f3/32.png) [@pretend\_my\_name\_is\_witty](https://boards.straightdope.com/u/pretend_my_name_is_witty)\ **Post date:** [October 18, 2004, 10:16pm UTC](https://boards.straightdope.com/t/question-about-sdmbs-no-homework-questions-policy-re-math-questions-specifically/269695/39 "2004-10-18T22:16:13Z")
Make that math_ **s** _ questions…sloppy typing! ---
**Author:** ![Thudlow\_Boink](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/thudlow_boink/32/320_2.png) [@Thudlow\_Boink](https://boards.straightdope.com/u/Thudlow_Boink)\ **Post date:** [October 18, 2004, 11:39pm UTC](https://boards.straightdope.com/t/question-about-sdmbs-no-homework-questions-policy-re-math-questions-specifically/269695/40 "2004-10-18T23:39:04Z")
> [@Pretend My Name Is Witty](#): > > So if you like math questions…and the kind people who replied to my thread did…then how are homework questions _“questions that absoultely no one cares about”_? I never said they were, necessarily! I said they _could be_, and _if_ they were, that was a reason not to post them. And I certainly didn’t mean to imply that _your_ question fell into that category. [Previous page](https://boards.straightdope.com/t/question-about-sdmbs-no-homework-questions-policy-re-math-questions-specifically/269695.md?page=1) [Next page](https://boards.straightdope.com/t/question-about-sdmbs-no-homework-questions-policy-re-math-questions-specifically/269695.md?page=3)