Find x Value for Cubic Function with TI-83

[cscott]

With my buddies TI-83 I've plotted a cubic function but am wondering how I can find the x value for a given y value.

 

[tmc]

Plot both f(x), the cubic, and g(x)=k, the constant function.

On top of "Trace", click on "Calc."

Go through the menu, look for intersect, and follow the instructions.

(The menu titles might not be exact, I don't have one in front of me)

 

[cscott]

I'm not sure if I totally follow but I assumed k = "y value". In my case this was 10^12. I don't really know what the intersect function is asking by "first curve?", "second curve?" and "guess?", but I clicked past them. With lower numbers it seems to work but with 10^12 I get "ERR: NO SIGN CHNG"

 

[z-component]

While viewing the graph, click "2nd" then "Trace" to bring up the Calc menu. Select "Value" and enter the x value for which you want the y value. You don't use "Intersection" unless you want the point at which two or more curves intersect and from what you're saying there's only one curve.

 

[cscott]

While viewing the graph, click "2nd" then "Trace" to bring up the Calc menu. Select "Value" and enter the x value for which you want the y value. You don't use "Intersection" unless you want the point at which two or more curves intersect and from what you're saying there's only one curve.

I want to do the opposite of what you described! I need the x value for a given y value. Thanks anyway.

 

[z-component]

Oh, there is no way that I know of using the original graphing software on the TI-83 to do that. I suppose the reason is because there can be multiple x values for a single y value in a function, not the other way around.

 

[tmc]

sure he can, simply by doing it my way...

Basically, he wants to solve for f(x)-k=0. So he can either plot f(x)-k, and look for where it crosses the x-axis (I don't remember if a function does that), or make two curves, f(x) and k, and find the x-coordinate of their intersection. Their intersection is where f(x)=k.

Another way would be to go into the solver (i don't remember where it is to be honest, hopefully you know...), which solves equations for you.

 

[cscott]

sure he can, simply by doing it my way...

Basically, he wants to solve for f(x)-k=0. So he can either plot f(x)-k, and look for where it crosses the x-axis (I don't remember if a function does that), or make two curves, f(x) and k, and find the x-coordinate of their intersection. Their intersection is where f(x)=k.

Another way would be to go into the solver (i don't remember where it is to be honest, hopefully you know...), which solves equations for you.

Yes, this worked. Thanks!

 

[z-component]

sure he can, simply by doing it my way...

Basically, he wants to solve for f(x)-k=0. So he can either plot f(x)-k, and look for where it crosses the x-axis (I don't remember if a function does that), or make two curves, f(x) and k, and find the x-coordinate of their intersection. Their intersection is where f(x)=k.

Another way would be to go into the solver (i don't remember where it is to be honest, hopefully you know...), which solves equations for you.

I understand where you're coming from now. That's a nice skill. I'll have to try that out sometime. :)