# TI-83 Question

## Main Question or Discussion Point

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."

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

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"

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.

z-component said:
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.

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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 dont 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 dont remember where it is to be honest, hopefully you know...), which solves equations for you.

tmc said:
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 dont 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 dont remember where it is to be honest, hopefully you know...), which solves equations for you.
Yes, this worked. Thanks!

tmc said:
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 dont 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 dont 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. :)