HP Forum Archive 20

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HP35S solve problem Message #1 Posted by Alessandro Castellani (Italy) on 31 Dec 2011, 9:35 a.m. I'm new in this forum and I have a problem on my 35S: when I try to solve for the expression x^2-4 I always find only the positive root (2) even if I give it two initial estimates like -3 and -1 very close to the negative root (-2). This happens only trying to solve the expression directly from the EQN list, if I define the expression in a program and I solve it giving the same initial estimates, solve result is fine. This is my operation sequence in RPN mode: I insert the expression in the EQN list: [EQN] X^2-4 [ENTER] [C] I store in the X variable the first estimate (-3) 3[+/-][STO][X][ENTER] I store in the X register the second estimate (-1) 1[+/-][ENTER] from EQN list I recall the expression and I solve it for the variable X: result is always 2 Could anyone explain me where I wrong ? Thanks in advance and my best wishes for an happy 2012 to everybody.

Re: HP35S solve problem Message #2 Posted by Dieter on 31 Dec 2011, 10:51 a.m., in response to message #1 by Alessandro Castellani (Italy) Alessandro, there's nothing wrong with your 35s. You just discovered a feature that is often overlooked: the 35s can solve equations symbolically. It does so if the variable appears only once in the equation. This also happens in your case. Here, the equation X^2 - 4 or X^2 - 4 = 0 is automatically transformed to X = SQRT(4) This exact solution is then evaluated by the 35s, using its regular functions. So the square root always returns a positive result. This also means that - in such a case - the two initial guesses do not matter at all, they are completely ignored. If you want the usual numerical solution by successive iteration, simply change the equation a bit so that the variable appears twice: X*X - 4 or X^2 - 4 = 0*X All this has been discussed before. For more information and further details, please take a look at this thread. Dieter Edited: 31 Dec 2011, 11:18 a.m.

Re: HP35S solve problem Message #3 Posted by Alessandro Castellani (Italy) on 2 Jan 2012, 3:37 a.m., in response to message #2 by Dieter Thank you very much Dieter. Alessandro

Re: HP35S solve problem Message #4 Posted by Paul Dale on 2 Jan 2012, 4:00 a.m., in response to message #2 by Dieter How fares the 35S with X! = 6 ?? Variable appearing exactly once but there is no inverse gamma function on the device that I'm aware of. - Pauli

Re: HP35S solve problem Message #5 Posted by Norman Dziedzic on 2 Jan 2012, 11:20 a.m., in response to message #4 by Paul Dale Without using the multiple variable trick in the equation it went for about 40 seconds and I canceled it. Using the equation X!=6+0*X It returned 3.000 (my setting Fix 3) after about 10 seconds.

Re: HP35S solve problem Message #6 Posted by Dieter on 2 Jan 2012, 3:06 p.m., in response to message #4 by Paul Dale Well, there are some restrictions so that not all cases with a single appearance of the variable are solved symbolically. For instance, consider periodic functions like sin or cos. An equation like SIN(X) = 0,5 will not be solved directly since there is an infinite number of solutions (30, 150, 390, 510, ... degrees). So the 35s will not simply set x = arcsin(0,5) and return 30 degrees. The case you mentioned X! = 6 cannot be solved directly since there is no inverse factorial or inverse Gamma function on the 35s. So the usual iterative approach is applied here. I tried various initial guesses between 0 and 20 - the 35s always came up with the correct result x = 3, usually within 4 - 10 seconds. No tweaking (...+0*X) was required. Dieter

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