Help with making a graph

[poochiecollins]

I apparently forget too much algebra. I'm trying to have one scale going from 4 to 9, with a value of 300 starting at four that decreases by 2/3 every point to 9. On my TI-83 calculator, I have set the window at:
Xmin= 4
Xmax= 9
Xscl=1
Ymin= 0
Ymax=350
Yscl= 10
Xres= 1
I made the function: y= 2/3x + 300

I graphed that, and a horizontal line formed at y=300. I want to be able to trace values of >300 part for every 1/10th of a point on the first scale, 4-9. What am I doing wrong?

[princecharles]

you can google a web based graph makin gsite..pretty good

[Blax0r]

I apparently forget too much algebra. I'm trying to have one scale going from 4 to 9, with a value of 300 starting at four that decreases by 2/3 every point to 9. On my TI-83 calculator, I have set the window at:
Xmin= 4
Xmax= 9
Xscl=1
Ymin= 0
Ymax=350
Yscl= 10
Xres= 1
I made the function: y= 2/3x + 300

I graphed that, and a horizontal line formed at y=300. I want to be able to trace values of >300 part for every 1/10th of a point on the first scale, 4-9. What am I doing wrong?

I don't think you're doing anything wrong.

I think it seems horizontal because of 1) the dimensions of your window and 2) the y-intercept just dominates the value of y for the domain you've chose for x.

I think the ti-83 has a feature that lets you get "y" for any "x" you input; check out the calc features (hit 2nd then trace). It may just be called "value".

Also, you could try adjusting your Xscl value to .1 (the scale of the x-axis), and the Ymin to 275.

[poochiecollins]

I don't think you're doing anything wrong. I think it seems horizontal because of 1) the dimensions of your window and 2) the y-intercept just dominates the value of y for the domain you've chose for x. I think the ti-83 has a feature that lets you get "y" for any "x" you input; check out the trace features (hit 2nd then trace). It may just be called "value". Also, you could try adjusting your Xscl value to .1 (the scale of the x-axis), and the Ymin to 275.

I want the Y value to decrease by 2/3 for every point of X. So, where X is 4, Y is 300, X=5 where Y=200, 6 to 133, 7 to 89, 8 to 59, and 9 to 40, rounding off decimals. When I trace, the relation is nothing like that.

Edit: I've corrected the slope to negative 2/3x, so at least there's the negative trend moving right that I want. This stuff is slowly coming back to me

[Blax0r]

I want the Y value to decrease by 2/3 for every point of X. So, where X is 4, Y is 300, X=5 where Y=200, 6 to 133, 7 to 89, 8 to 59, and 9 to 40, rounding off decimals. When I trace, the relation is nothing like that.

Edit: I've corrected the slope to negative 2/3x, so at least there's the negative trend moving right that I want. This stuff is slowly coming back to me

ohh yea, flipping the slope should fix that. I missed that part in your original message. Also, I think you can get the ti-83 to tabulate the values (should be easy enough to google) so you don't have to worry about using the tracing tool on the graph screen.

also, i recommend getting the latest version of the ti-89; probably will cost around $150, but it's really worth it.

[poochiecollins]

ohh yea, flipping the slope should fix that. I missed that part in your original message. Also, I think you can get the ti-83 to tabulate the values (should be easy enough to google) so you don't have to worry about using the tracing tool on the graph screen. also, i recommend getting the latest version of the ti-89; probably will cost around $150, but it's really worth it.

Oh, I guess I didn't make it clear in my last post that the graph's still not doing what I want. E.g. when I trace from 4 to 5 on the x-axis, the y-value goes from 300 to 297-something.

[princecharles]

How did the webbased idea work out>??;

[poochiecollins]

How did the webbased idea work out>??;

I've Googled for help a few times. One thing I found the other day is that I think to get the downward-sloping curve that the line should be, the slope would have to be a fractional power, like the cube route of two or something.

If there's an Internet tool that could create a line based on the relationship between multiple plot points, that'd be great and would also answer what I'm looking for, if anyone's aware of such or knows what to search.

[dr_wolf]

First time you need to make 2 axes x,y and you you have a function f(x)=y so you need only to calculate the values >300,and after that compare the values.

[poochiecollins]

First time you need to make 2 axes x,y and you you have a function f(x)=y so you need only to calculate the values >300,and after that compare the values.

Why, since nothing goes over 300, within my parameters?