Math Forum Discussions

Lynn Fisher

Posts: 16
Registered: 12/6/04

Unwanted (?) Imaginary Roots from TI-85
Posted: Nov 21, 1997 12:43 AM

Plain Text

For anyone who uses TI-85'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 TI-85 does this...I've noticed that, on the other hand, the TI-86
is well-behaved 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