Hello,
Can owners of the HP20b confirm this, or at least explain to me where I am wrong - I know I am, because I know HP is not!
1 - Turn off HP20b
2 - Press&Hold [N]+[Amort], press [ON/CE] (this will reset the calc to defaults, not necessary, but to make sure we are starting from scratch)
3 - [fn],[Mode], set to RPN
4 - [ON/CE]
5 - [fn],[Data]
6 - enter the following data:
X(1) = 0
Y(1) = 1
X(2) = 1
Y(2) = 2
X(3) = 2
Y(3) = 4
X(4) = 3
Y(4) = 8
(in other words, Y = 2^X)
7 - [fn],[Stats]
8 - on "2 Vars", scroll down to "Predictions"
9 - [INPUT]
10 - [INPUT] until b*a^X
11 - scroll to check that a = 2 and b = 1 (correct)
12 - scroll to "Pred X", enter "3", [INPUT]
13 - scroll to "Pred Y", [=] (returns 8, good)
14 - now scroll back to "Pred X", [=] (returns 2.81, what the heck?)
2.81 is the predicted X for Y=7. If you go back and forth predicting X and Y, Y goes down in steps of "1"
Same happens with the b*e^(aX).
Linear data (resulting from a a*X+b) works fine (you can go back and forth from predicting X and Y, and the values stay put).
After testing with data that correlates to each of the prediction functions, it looks to me that:
b*a^X : weird
b*e^(aX) : weird
a*X+b : ok
a/X+b : weird
b*X^a : weird
a*ln(X)+b : ok
Where am I messing up? I hope I am, because I have no JTAG cable to update the calc firmware...
Best regards,
Joao S Veiga