HP 39gs Master Manual page 136

Cubic
Exponent
Trigonometric
There is no easy way to retrieve the coefficients in the trigonometric equation. The 'simplest' way is to firstly
transfer it to the Function aplet by entering
pressing . If you now change to the
appear in the view. You can then
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ideal. It would be simpler to write them down and re-type them when required!
Correct interpretation of the PREDX function
The function in the
PREDX
Algebraically it simply reverses the line of best fit. For example, the equation Y ˆ = ⋅
(Y − 1.1662)
would use
X ˆ =
0.8199
Whether this is mathematically correct depends on how you interpret the
you interpret it to mean "give me an x value which, if used in the
value", then it is correct. However, it should not be interpreted to mean "predict an x value based on this y
value" as most people might.
The reason for not using the second interpretation is that the results it gives would then be incorrect. The line of
best fit (unlike the correlation) changes as the independent and dependent variables swap roles and can't be
simply algebraically reversed in this way. It should not be thought that the hp 39gs & hp40gs are unusual in
this odd interpretation. Most calculators' equivalent of the
function behave in the same manner.
The formula for the slope
S
the formula: b = .
xy
( )
2
S
x
-
a*X^3+b*X^2+cX+d
⎡ ⎤ ⎡ 0
a
⎢ ⎥ ⎢
b ⎢ ⎥ ⎢ 1
=
⎢ ⎥ ⎢ 8
c
⎢ ⎥ ⎢
⎢ ⎥ ⎢ 27 9 3 1 ⎥
d ⎣ ⎦ ⎣
-
b*EXP(m*X)
-
a*SIN(b*X+c)+d
F1(X)=PREDY(X)
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menu is not really a way of predicting x values despite its name.
MATH
to predict the X values.
in the line of best fit ˆ =
b
−1
0 0 1 ⎤ ⎡ PREDY (0) ⎤
⎥ ⎢
1 1 1 ⎥ ⎢ PREDY (1)
×
4 2 1 ⎥ ⎢ PREDY (2) ⎥
⎥ ⎢
⎢ PREDY (3) ⎥ ⎦
⎦ ⎣
b =
PREDY(0)
m =
PREDY(1)/PREDY(0)
into the Function aplet, highlighting it and then
view and type
F1(QUOTE(X))
it and edit out any coefficients you want. Clearly this is not
PREDY
PREDX
a + bx is given by
y
136
⎥
⎥
⎥
then the equation will
0 8199 X +1 1662 earlier
⋅
function. If, as HP intended,
PREDX
function, would give me this y