HP 15c User Manual
Page 173
Appendix: Accuracy of Numerical Calculations
173
Keystrokes
Display
÷
021- 10
|K
022- 43 36
|(
023- 43 33
÷
024- 10
|n
025- 43 32
´b.9
026-42,21,.9
¤
027- 11
lV
028- 45 25
|(
029- 43 33
÷
030- 10
®
031- 34
|K
032- 43 36
÷
033- 10
´V
034- 42 25
v
035- 36
´}
036- 42 30
”
037- 16
´}
038- 42 30
|n
039- 43 32
|¥
The method uses d = b
2
− ac.
If d < 0, then the roots are a complex conjugate pair
a
d
i
a
b
.
If d ≥ 0, then the roots are real numbers x and y calculated by
)
(
sgn b
d
b
s
a
s
x
/
0.
s
if
0
0
s
if
/ s
c
y
The s calculation avoids destructive cancellation.
When a = 0 ≠ b, the larger root x, which should be ∞, encounters division by zero (Error 0)
that can be cleared by pressing ) three times to exhibit the smaller root y correctly
calculated. But when all three coefficients vanish, the Error 0 message signals that both
roots are arbitrary.