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Unwanted (?) Imaginary Roots from TI85
Posted:
Nov 21, 1997 12:43 AM


For anyone who uses TI85's: Have you noticed that entering something like
(8)^(2/3)
results in an imaginary number? It's one of the cube roots of 64, as is the expected "real" answer, 4. When I first noticed this a year or so ago, I thought it quirky but a great opportunity to talk to students about why it's a correct answer, even though it's not what we expect. And, the calculator's behavior can be circumvented by entering
(8)^(2)^(1/3) or, vice versa (8)^(1/3)^(2)
There's another development: the calculator won't produce a graph of, or evaluate, the derivative of a cube root function over the values for which the argument of the cube root is negative. If you've never noticed this, try graphing or evaluating the derivative of y = x^(1/3) using the "der1" function.
The "nder" , or "numerical derivative" works okay. But I'm curious as to why the TI85 does this...I've noticed that, on the other hand, the TI86 is wellbehaved on this matter, so somebody at TI has noticed, I suppose. Perhaps I should be sending this post to TI...anybody got the address?
Lynn Fisher Woodstock Union HS Woodstock VT



