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gnkarthik
Junior Member

USA
3 Posts

 Posted - 10/05/2007 :  09:42:05 I have an equation I am trying to feed into excel.E = a*p^c +b*p^dI want the equation to be transformed such that I can express p in terms of E. Any expertise in the forum??

tkhunny

USA
1001 Posts

 Posted - 10/05/2007 :  11:02:05 There is expertise, but you are not going to like the answer.There is no such general solution. For specific values of c and d, there can be solutions, but not for arbitrary c and d.So, there are a couple of ways to go...1) What is the nature of the process. What are c and d likely to be? If they are sufficiently consistent and useful, there may be an answer to your question. For example, if c is always twice d (c = 2d), that would be useful.2) In MS Excel you can use the "Goal Seek" function for one-time solutions or you can write a Visual Basic function to handle larger numbers of cases.Where does that leave us?

gnkarthik
Junior Member

USA
3 Posts

 Posted - 10/05/2007 :  12:22:51 Thanks for the quick turn around.I know c and d are constants. So are a and b. I know the values for all a,b,c and d. Since this is a speicific problem, I can take ratios c/d and a/b.p and E are variables.Where do I go from there?

gnkarthik
Junior Member

USA
3 Posts

 Posted - 10/05/2007 :  14:34:01 OK.My question would nowbe... What is a good resource to learn about multinomial equations??If someone wants to help me......here is my real problema =1124/211000b=0.37495c=-0.10853d=-0.63541E varies from 0.0164637 to 0.00103254. Please look at the table below.I need to design for a particular value of p for multiple situations. Goalseek in MS excel is an additional step which I preferably want to avoid (because it is a manual operation unless I write a macro). p E100 0.0164637112.202 0.0155073125.893 0.0146157141.254 0.0137846158.489 0.0130095177.828 0.0122867199.526 0.0116125223.872 0.0109835251.188 0.0103964281.838 0.00984845316.227 0.00933684354.813 0.00885904398.107 0.00841271446.683 0.00799564501.187 0.00760581562.34 0.0072413630.956 0.00690038707.944 0.0065814794.327 0.00628283891.249 0.00600327999.998 0.00574141122.02 0.0054961258.92 0.005265921412.53 0.005050121584.89 0.004847611778.27 0.004657471995.26 0.004478862238.72 0.004310992511.88 0.004153122818.37 0.004004583162.27 0.003864723548.12 0.003732953981.06 0.003608734466.82 0.003491545011.85 0.003380925623.39 0.003276416309.55 0.003177627079.43 0.003084157943.25 0.002995658912.47 0.002911799999.96 0.0028322611220.1 0.0027567812589.2 0.0026850814125.3 0.0026169115848.9 0.0025520517782.8 0.0024902819952.6 0.0024313922387.2 0.0023752125118.8 0.0023215628183.8 0.0022702831622.8 0.0022212235481.4 0.0021742539810.8 0.0021292244668.4 0.0020860250118.8 0.0020445556234.3 0.0020046863095.9 0.0019663470794.8 0.0019294179433.1 0.0018938389125.4 0.00185951100000 0.00182638112202 0.00179437125893 0.00176342141255 0.00173346158490 0.00170444177829 0.00167631199528 0.00164902223874 0.00162253251191 0.00159679281841 0.00157176316231 0.00154741354817 0.00152371398111 0.00150061446688 0.0014781501193 0.00145614562348 0.00143471630965 0.00141378707954 0.00139333794338 0.00137333891262 0.001353781.00E+06 0.001334641.12E+06 0.001315911.26E+06 0.001297561.41E+06 0.001279581.58E+06 0.001261961.78E+06 0.001244682.00E+06 0.001227722.24E+06 0.001211092.51E+06 0.001194752.82E+06 0.001178713.16E+06 0.001162963.55E+06 0.001147483.98E+06 0.001132264.47E+06 0.00111735.01E+06 0.00110265.62E+06 0.001088136.31E+06 0.00107397.08E+06 0.001059897.94E+06 0.001046118.91E+06 0.00103254

tkhunny

USA
1001 Posts

 Posted - 10/05/2007 :  15:54:30 I set up a simple example like this:a 0.0053270 b 0.3749500 c (0.1085300)d (0.6354100)E 0.0164637 p 188.2338165a is in a1b is in a2etc.The value for a is in b1The value for b is in b2etc.The formula for p in b6 is =FindP(b1,b2,b3,b4,b5) FindP is defined in Visual Basic as:Function FindP(aVal, bVal, cVal, dVal, EVal) Static HoldVal, BackVal, TolVal, FofP, FPrimeofP As Double TolVal = 0.000001 BackVal = EVal / (aVal + bVal) ' Surrogate Initial p-value For CountVal = 1 To 15 FofP=EVal-(aVal*(BackVal^cVal))-(bVal*(BackVal^dVal)) FPrimeofP=0-(aVal*(cVal*(BackVal^(cVal-1))))-(bVal*(dVal*(BackVal^(dVal-1)))) HoldVal=BackVal-FofP/FPrimeofP If Abs(HoldVal - BackVal) < TolVal Then Exit For Else BackVal = HoldVal End If Next FindP = HoldValEnd FunctionIt's just Newton's method and should work most of the time. I found 15 iterations to be sufficient im many cases.Be warned. I don't get anywhere near your values. I would have to question the values you provided.I get E = 0.164637 and p = 188.233816Be additionally warned, I don't know what you are doing, here. I know only that you asked a decent question, although you could have provided better information. If you are doing something dangerous, don't blame me. I can't be held responsible for your coding errors or for your behavior.Note: This is one of the reasons tutors should offer only hints, rather than complete solutions. Liability. Just something to think about. It's a crazy world we live in.

Jason

USA
1235 Posts

 Posted - 10/11/2007 :  12:11:09 I get what tkhunny getsWhen E = .0164637 then p = 188.233816465834272926When E = .00103254 then p = 4495384.844865500925
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