Question about SOLVE on hp 35s « Next Oldest | Next Newest »

 ▼ Matt Member Posts: 56 Threads: 9 Joined: Jan 2007 08-04-2007, 07:30 AM ```Using EQN, I have entered two equations:- (X+2)*(X-2) and X^2-4 Next, I input 3 STO X EQN (then 'select' (X+2)*(X-2)) SOLVE X And the output is X=2.0000 I repeat the process with X^2-4 and get the same result. Now, I input -3 STO X and repeat the above steps. For (X+2)*(X-2) I get the result X=-2.0000 as expected. But for X^2-4 I get X=2.0000. I cannot seem to obtain X=-2.0000 as a root of X^2-4. Why is this? ``` ▼ Antonio Maschio (Italy) Senior Member Posts: 416 Threads: 78 Joined: Mar 2006 08-04-2007, 07:36 AM Same behavior of the HP-33s. Look in the Forum. -- Antonio ▼ Matt Member Posts: 56 Threads: 9 Joined: Jan 2007 08-04-2007, 08:35 AM ```Thanks. I never had a 33 so I wasn't aware of the behaviour. ``` ▼ Gene Wright Posting Freak Posts: 1,545 Threads: 168 Joined: Jul 2005 08-04-2007, 09:35 AM See example 2 in the 35s learning module "Formula solver part 2" found here: When a variable shows up only once in an equation such as X^2-4=0, the solver finds a direct solution if possible and ignores any user supplied guesses. That's a carryover from the 33s. Example 2 suggests the workaround. Sondre K. Selnes Junior Member Posts: 7 Threads: 0 Joined: Jul 2007 08-04-2007, 09:20 AM Tried your equation / SOLVE at HP-15C, which gives X= 2.0000 and X= -2.0000. Maybe the correct answer of HP35s is found in another register... Edited: 4 Aug 2007, 10:21 a.m. Karl Schneider Posting Freak Posts: 1,792 Threads: 62 Joined: Jan 2005 08-04-2007, 12:53 PM Matt -- Here's the link to the thread in the Forum Archives: The link is my proposed solution (obviously not adopted); scroll to the top to find a very similar discussion for the HP-33S. It turns out that the "direct solution" logioc seems to have been lifted from the HP-17B/HP-27S from 1988; I didn't know that at the time. -- KS ▼ Gene Wright Posting Freak Posts: 1,545 Threads: 168 Joined: Jul 2005 08-04-2007, 01:02 PM And, again, I am still sorry for the shortness of that title back then in my response to you. :-) This "direct solve" approach is considered a feature. You don't take features away...anyone knows that. :-) Short answer: if you have a polynomial that is missing a term, add the term with a 0 coefficient. X^2-4=0 would be entered as x^2+0x-4=0. Then the solver will not try a direct solution. That is the way it is. Matt Member Posts: 56 Threads: 9 Joined: Jan 2007 08-05-2007, 06:14 AM ```Thanks for the links. I'll have a good at all the training modules. ```

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