Question about SOLVE on hp 35s



#2

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?


#3

Same behavior of the HP-33s.

Look in the Forum.

-- Antonio


#4

Thanks.
I never had a 33 so I wasn't aware of the behaviour.

#5

See example 2 in the 35s learning module "Formula solver part 2" found here:

Link


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.

#6

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.

#7

Matt --

Here's the link to the thread in the Forum Archives:

http://www.hpmuseum.org/cgi-sys/cgiwrap/hpmuseum/archv014.cgi?read=62665#62665

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


#8

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.

#9

Thanks for the links.
I'll have a good at all the training modules.

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