HP 35S EQN solve only finding one solution

[Dieter]

(09-25-2017 08:56 PM)Trond Wrote:  Is anyone else having this problem?
Try a simple equation with two possible solutions for X, e.g. X^2-4=0.
My 35S seems to only be able to find one solution (2), while my 32SII finds either 2 or -2, depending on my starting values. I do this exactly the same for both; if I store negative values for x and in the x register, then 32SII finds the negative solution of -2, while positive gives 2. But 35S finds only 2, seemingly no matter what I do.

Yes, the 35s is far more intelligent than the 32SII in this regard. Since X only occurs once in the equation it can provide a symbolic (!) solution: x²–4 = 0 => x = √4 which is +2 for the 35s. Just the same as if you had pressed 4 √x.

That's why a solution is returned immediately, regardless of the two initial guesses in the variable X and the stack's x-register.

Of course you can also solve without this feature: simply add another X somewhere, e.g. by a simple X^2 + 0xX – 4. The "=0" part is not required. Now 0 STO X –5 SOLVE X returns -2. You will also notice that it takes a short moment before the solution is displayed. Just because now the result is not calculated directly but – as usual – approximated by iteration.

Dieter