HP-50g plots of cube roots (no negative domain displayed)

[Dieter]

Cliff,
as you can see you posted this message twice. Please delete the other one. To do so, log in and use the delete button below your post (the one with the red X).

(03-19-2018 04:35 PM)Cliff Stamp Wrote:  If I plot y1(x)=x^(1/3), I get no negatives. I did a bit of searching and turned up :

http://www.hpmuseum.org/cgi-sys/cgiwrap/...ead=141921

The solution there was kind of awkward as if you want to plot say :

y1(x)=x^(5/3)

you have to plot :

y1(x) = (cube root(x))^5

I do not have a 50g, but this behaviour is the way I would expect a decent calculator to react: While x^(1/3) mathematically is the same as the cube root of x, it isn't for a calculator with finite precision: for the cube root of –8 it would calculate –8^0,333333333333 which is not the same as –8^(1/3). So the solution is complex, as the 12-digit-rounded value of 1/3 equals the 3,000000000003rd root of –8.

Then you wrote:

(03-19-2018 04:35 PM)Cliff Stamp Wrote:  ...as both

y1(x)=x^(5/3)

and

y1(x) = cube root(x^5)

Fail to show the negative domain as the calculator is generating the complex roots when x < 0.

It should be clear now why the first method does not work the way you want to.

But I wonder why

y1(x) = cube root(x^5)

should not work while

y1(x) = (cube root(x))^5

gives the desired result.

Are you sure about this?

Dieter