Bertfatal Posted March 21, 2007 Share Posted March 21, 2007 My son has been given this to solve and it is beyond me! x3 + x2 = 30 (x cubed plus x squared equals 30) How do you calculate x? Bertfatal Link to comment Share on other sites More sharing options...
Stationary M25 Traveller Posted March 21, 2007 Share Posted March 21, 2007 It's a quadratic equation (I think). Can't remember - it has been over 30 years since I did these !! ****************** And you run and you run to catch up with the sun, but it's sinking... And racing around to come up behind you again. photos Link to comment Share on other sites More sharing options...
Stationary M25 Traveller Posted March 21, 2007 Share Posted March 21, 2007 It's a bit less than 3 !! ****************** And you run and you run to catch up with the sun, but it's sinking... And racing around to come up behind you again. photos Link to comment Share on other sites More sharing options...
Stationary M25 Traveller Posted March 21, 2007 Share Posted March 21, 2007 ...... in fact more than 2.8 and less than 2.82 !! Mindlessly playing with calculator ****************** And you run and you run to catch up with the sun, but it's sinking... And racing around to come up behind you again. photos Link to comment Share on other sites More sharing options...
Roadsport06 Posted March 21, 2007 Share Posted March 21, 2007 x= -1.9 + 2.7i, -1.9 - 2.7i or 2.81 BRG and Yellow nose VU06 XZX Edited by - JamesHarding on 21 Mar 2007 16:16:43 Link to comment Share on other sites More sharing options...
Bertfatal Posted March 21, 2007 Author Share Posted March 21, 2007 at the moment my son has got 2.80712644864 and it = 29.99999994 so he is 0.00000006 out do you think he could find away to make it exactly 30? Link to comment Share on other sites More sharing options...
Roadsport06 Posted March 21, 2007 Share Posted March 21, 2007 I don't think that this one solution gives a fraction. I think that it shoud go to a few sig figures anyway. Remember that two other solutions are imaginary numbers too. BRG and Yellow nose VU06 XZX Link to comment Share on other sites More sharing options...
Bertfatal Posted March 21, 2007 Author Share Posted March 21, 2007 His teacher has told him that he will give him £5 if he gets an exact answer, do you think he is winding him up? Bertfatal Link to comment Share on other sites More sharing options...
Steve T Posted March 21, 2007 Share Posted March 21, 2007 2.80712645078456 as near as dammit !! this gives 12 9's btw Steve 2,500 miles so far and counting ! My Pictures Edited by - Steve T on 21 Mar 2007 16:30:07 Link to comment Share on other sites More sharing options...
Roadsport06 Posted March 21, 2007 Share Posted March 21, 2007 Perhaps the teacher is expecting the imaginary answers too. If not then the answer could almost go on forever, and would require plenty differentiation. I better carry on with marking some physics homework now :) BRG and Yellow nose VU06 XZX Link to comment Share on other sites More sharing options...
Bertfatal Posted March 21, 2007 Author Share Posted March 21, 2007 we now got 2.80712645500001 which is 0.00000001 out! Link to comment Share on other sites More sharing options...
Stationary M25 Traveller Posted March 21, 2007 Share Posted March 21, 2007 Close - but no cigar ! ****************** And you run and you run to catch up with the sun, but it's sinking... And racing around to come up behind you again. photos Link to comment Share on other sites More sharing options...
Steve T Posted March 21, 2007 Share Posted March 21, 2007 I think you may have an error on the 9th digit after decimal point, see my answer a few posts ago, it is more accurate PS unless I have made a mistake then appologies Steve 2,500 miles so far and counting ! My Pictures Link to comment Share on other sites More sharing options...
Dai-X Posted March 21, 2007 Share Posted March 21, 2007 Close - but no cigar !Isn't that how Bill Clinton recently described his relationship with Hilary? Sorry - I'll get my coat and let you get back to the Maths, Ben Link to comment Share on other sites More sharing options...
Bertfatal Posted March 21, 2007 Author Share Posted March 21, 2007 after much calculator pressing we have got an exact answer acording to our calculator it was x= 2.8071264508 please confirm!!! ( that was a waste of time) Link to comment Share on other sites More sharing options...
Steve T Posted March 21, 2007 Share Posted March 21, 2007 2.80712645078456 as near as dammit !! this gives 12 9's btw your calculator has rounded a bit Steve 2,500 miles so far and counting ! My Pictures Link to comment Share on other sites More sharing options...
Stationary M25 Traveller Posted March 21, 2007 Share Posted March 21, 2007 Now to get back to my customers !! ****************** And you run and you run to catch up with the sun, but it's sinking... And racing around to come up behind you again. photos Link to comment Share on other sites More sharing options...
AdamQ Posted March 21, 2007 Share Posted March 21, 2007 2.80712645078457 gives 30.0000000000 in Excel (so, allowing for the Microsoft fudge factor, that's probably about 28.4 ❗) Adam Link to comment Share on other sites More sharing options...
millsn Posted March 21, 2007 Share Posted March 21, 2007 It's a cubic or polynomial not a quadratic. Try this link, it might help it's too late for me to revise this now, sorry. http://helm.lboro.ac.uk/documents/3_3_solving_polynomial_equatns.pdf Link to comment Share on other sites More sharing options...
millsn Posted March 22, 2007 Share Posted March 22, 2007 I don't think nyou can arrive at a solution to this problem very easily at all. I'm not sure what age your son is but I would suspect that the best approach is to plug a few numbers inot the calculator until you can sketch the graph of the function and use the graph to approximate the result desired. I''ve tried a few other routes to solve the equation and failed having been at it some time now. Link to comment Share on other sites More sharing options...
Neil A Posted March 23, 2007 Share Posted March 23, 2007 I agree. Draw a graph. Where the line intersects the Y axis is your answer. Simplist solution I think. Fellas, smoke me a kipper, I'll be blatting for breakfast. Link to comment Share on other sites More sharing options...
AdamQ Posted March 23, 2007 Share Posted March 23, 2007 I agree about drawing a graph to get an initial estimate and then you can use Newton-Raphson to get a more accurate value for the real root. Re-arrange the equation to xxx + xx - 30 = 0 = f(x). Then f'(x) = 3xx + 2x and then you can use x(n+1) = x(n) - f(x(n))/f'(x(n)) to get successively closer approximations. Once you've got the real root, you can factorise to a linear and quadratic form and then calculate the complex roots. Or you can just use the Excel Goal seek function (which probably does something similar to the above anyway!). A far more taxing problem as an aside: does anybody know how to search for pdf files containg certain text in XP?? Regards, Adam Link to comment Share on other sites More sharing options...
MTH Posted March 23, 2007 Share Posted March 23, 2007 It's a known problem with XP for which there's a registry hack (I assume you're on the latest SP). Link to comment Share on other sites More sharing options...
AdamQ Posted March 23, 2007 Share Posted March 23, 2007 Thanks MTH. I've tried both the methods in the article, but with no success I'm afraid. I'm wondering if I have to wait for Windows to complete the indexing process before it will work - that would make sense to me. In the meantime, I'm trying out some freeware called File Hand Search - we'll see if that works. Apologies to all for going off at a tangent from the original post. Adam Link to comment Share on other sites More sharing options...
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