Graphing a function in the window?

**krzyrice** · January 30th 2010, 05:14 PM

I need help with this problem. So do I start out with plugging in random values for x? Such as -2, -1, 0, 1, 2 and so on. And what do they mean by the "Graph the following function in the window -10 < x < 10, -10 < y < 10?" Thank you.

**Prove It** · January 30th 2010, 05:19 PM

Originally Posted by krzyrice

I need help with this problem. So do I start out with plugging in random values for x? Such as -2, -1, 0, 1, 2 and so on. And what do they mean by the "Graph the following function in the window -10 < x < 10, -10 < y < 10?" Thank you.

Expand it out, so that you have

$y = -\frac{1}{900}x^2 + \frac{10}{9}x$ .

You should know how to graph quadratics by finding $x$ and $y$ intercepts and turning points.

**krzyrice** · January 30th 2010, 05:25 PM

So like I said do I just plug in random values of x now and find the y values of it? Then I graph the equation and find the average rate of change to see if it is a line or not? Thanks for helping me so far.

**Prove It** · January 30th 2010, 05:50 PM

Is a Quadratic function ever a line?

Like I said, the easiest way to sketch a graph is to find the $x$ and $y$ intercepts and the turning point. This gives you an idea of the shape of the graph.

And then yes, you can plot points to make your graph a bit more accurate.

**krzyrice** · January 30th 2010, 05:57 PM

Nope a quadratic it's never a line. But What I don't get is what is the -10 < x < 10 and -10 < y < 10 for? What am I suppose to do with those?

**mr fantastic** · January 30th 2010, 06:09 PM

Originally Posted by krzyrice

Nope a quadratic it's never a line. But What I don't get is what is the -10 < x < 10 and -10 < y < 10 for? What am I suppose to do with those?

You are being told what scale to use on your x and y-axes (potentially, you're meant to graph this on a Graphing calculator and you're being told what window to use).

**skeeter** · January 30th 2010, 06:36 PM

screenshots ...

**krzyrice** · January 30th 2010, 07:27 PM

I thought the shape was suppose to be a parabola. Dang this problem is giving me a hard time. Sorry I don't have a graphing calculator so I don't really know how to solve this. And my teacher makes us write out the answer and explain how we got it also.

**skeeter** · January 30th 2010, 07:46 PM

Originally Posted by krzyrice

I thought the shape was suppose to be a parabola. Dang this problem is giving me a hard time. Sorry I don't have a calculator so I don't really know how to solve this. And my teacher makes us write out the answer and explain how we got it also.

the graph is a parabola ... you're only looking at a very small section of the graph in the given window.

here's how it looks in an expanded window ...

**skeeter** · January 30th 2010, 07:48 PM

free download and easy to use ...

Graph

**krzyrice** · January 30th 2010, 08:59 PM

I just tried graphing it on a graphing calculator and it gave me this:

**HallsofIvy** · January 31st 2010, 02:34 AM

Originally Posted by krzyrice

I just tried graphing it on a graphing calculator and it gave me this:

Okay, what window did you use? What does the graph look like if you set the window to $-10\le x\le 10$ , $-10\le y\le -10$ as in your problem?

**skeeter** · January 31st 2010, 04:48 AM

Originally Posted by krzyrice

I just tried graphing it on a graphing calculator and it gave me this:

that is not the function the exercise gave you ... $y = -x\left(\frac{x-1000}{900}\right)$

looks like you graphed $y = x^2$

**krzyrice** · January 31st 2010, 10:41 AM

In the calculator I typed in -x(x-1000/900). Should I have typed it differently? I also tried your way -x(x-1000)/900 and got a straight line. And even if I select zoomfit it would still show a straight line.

**krzyrice** · January 31st 2010, 10:45 AM

Originally Posted by HallsofIvy

Okay, what window did you use? What does the graph look like if you set the window to $-10\le x\le 10$ , $-10\le y\le -10$ as in your problem?

This is what it looks like when I use $-10\le x\le 10$ , $-10\le y\le -10$

Also I input the equation in the calculator as -x((x-1000/900))

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