Plotting a function not working correctly

[Ollie]

Hey!
I've been experimenting and finding roots of polynomials graphically but today I came across a weird glitch.
I tried to plot the following function: 0.1x^5 + 1.3x^4 - x^3 - 15x^2 - 8x - 1, a function that has 5 different roots.
The problem is that I cannot see the line that is supposed to cross the x-axis for the first time, all I see are very sparsely scattered dots instead of a continuous line.
Has anyone else come across such a problem before on the Prime? If so, have you found a fix for it?

[Han]

(04-13-2015 03:46 PM)Ollie Wrote:  Hey!
I've been experimenting and finding roots of polynomials graphically but today I came across a weird glitch.
I tried to plot the following function: 0.1x^5 + 1.3x^4 - x^3 - 15x^2 - 8x - 1, a function that has 5 different roots.
The problem is that I cannot see the line that is supposed to cross the x-axis for the first time, all I see are very sparsely scattered dots instead of a continuous line.
Has anyone else come across such a problem before on the Prime? If so, have you found a fix for it?

Check your plot settings. Press [Shift][Plot] to open the plot settings, and look at the second page of settings (use the menu option at the bottom of the screen to get to different pages of settings).

[John Colvin]

(04-13-2015 03:46 PM)Ollie Wrote:  Hey!
I've been experimenting and finding roots of polynomials graphically but today I came across a weird glitch.
I tried to plot the following function: 0.1x^5 + 1.3x^4 - x^3 - 15x^2 - 8x - 1, a function that has 5 different roots.
The problem is that I cannot see the line that is supposed to cross the x-axis for the first time, all I see are very sparsely scattered dots instead of a continuous line.
Has anyone else come across such a problem before on the Prime? If so, have you found a fix for it?

Plots ok on my Prime. You have to zoom in on the x-axis quite a bit to see where the curve crosses the axis. As Han suggested, check your plot settings to make sure your plot style is either fixed step segments or Adaptive-not fixed step dots.

[Ollie]

Unfortunately I'm still having problems with this.
I can see all the other intersections but there's one missing at x ~ -12.91

Settings:

The more I go up the more lonely single dots I can see on the curve. :/

[Tugdual]

(04-18-2015 11:43 AM)Ollie Wrote:  Unfortunately I'm still having problems with this.
I can see all the other intersections but there's one missing at x ~ -12.91

Settings:

The more I go up the more lonely single dots I can see on the curve. :/

Nothing really new to me here... Note that the 50g does a perfect job here, just much slower.

[Ollie]

(04-18-2015 12:05 PM)Tugdual Wrote:  Nothing really new to me here... Note that the 50g does a perfect job here, just much slower.

Are you trying to say that this is intended and not a bug/glitch of some sort?
Because if it is then that's just outright dumb, even my TI84 can graph the function perfectly without leaving any lonely dots on the curve.

[Tugdual]

(04-18-2015 12:09 PM)Ollie Wrote:  

(04-18-2015 12:05 PM)Tugdual Wrote:  Nothing really new to me here... Note that the 50g does a perfect job here, just much slower.

Are you trying to say that this is intended and not a bug/glitch of some sort?
Because if it is then that's just outright dumb, even my TI84 can graph the function perfectly without leaving any lonely dots on the curve.

I don't think it is intended, I'm just meant that it is a glitch I have seen a long time ago and was never fixed.

[Marcel]

Hi,
If you use the Advanced Graphing, you will see the correct graph, but this is very long and not very practicaly. All my TI calculator can graph this fonction correctly!
Marcel

[CR Haeger]

I'll ch k this out when I get home.

In the meantime, try out solve(), zeros() and fsolve() in CAS which should all give you solutions quickly.

[Alberto Candel]

Hi,

It looks like to be able to see the whole part of the graph that includes all roots, you need to set the x-range to include all roots, and the y-range to be at least the max/min of your polynomial over the selected x-range. In this case x from -15 to 15 and y from -100 to 100 seem to work. An estimate for the x-range can be easily found (Lagrange).

[Ollie]

(04-18-2015 04:52 PM)Alberto Candel Wrote:  Hi,

It looks like to be able to see the whole part of the graph that includes all roots, you need to set the x-range to include all roots, and the y-range to be at least the max/min of your polynomial over the selected x-range. In this case x from -15 to 15 and y from -100 to 100 seem to work. An estimate for the x-range can be easily found (Lagrange).

Ooh, It works!
It seems like the calculator sort of dismisses the parts that go outside your x- and y-ranges but setting the correct ranges fixes it.
Kinda annoying, but I guess we can live with it.
Thanks!

[Alberto Candel]

The max of your polynomial over the range of the roots (like -15<x<3) is not larger than 3000, but the plot shows the relevant part (the 5 roots) with a y-range setting of about +/-100.