Need help for Math.

puppyloverlauren • 3 October 2012 at 8:18 PM

How do you explain how to write an inequality that is modeled by a graph?

silver_winged • 3 October 2012 at 8:19 PM

@puppyloverlauren

Just to clarify: you have a graph, and you want to write the equation/inequality?

puppyloverlauren • 3 October 2012 at 8:24 PM

@silver_winged

You have to write an inequality equation thing from a graph. So the graph is already there, and you have to make an inequality equation.

sunshinecat • 3 October 2012 at 8:27 PM

@puppyloverlauren Linear equations?

Find the gradient of the line (rise/run) and the y-intercept. Substitute into ax + b (a=gradient, b=y-intercept).

Then for the inequality if the shaded part is below the line it will be y < and if the shaded part is above it will be >.

Edit: Some signs don't seem to work on EC. But this isn't what you were searching for anyway.

puppyloverlauren • 3 October 2012 at 8:29 PM

@sunshinecat no just like a horizontal lined ones like this:

<--0--1--2--3--4--5> stuff like this

sunshinecat • 3 October 2012 at 8:31 PM

@puppyloverlauren Wait, huh? That isn't a graph...unless you want to draw the graph from a list of numbers then find the inequality.

I'm from New Zealand, so we'll have a different curriculum than in the US or Europe, so we might not do the same stuff you do.

Is it like this? http://img.sparknotes.com/figures/7/7b1d69c7307282c81989a3e34b51a45c/xless7_union_xgeq11.gif

silver_winged • 3 October 2012 at 8:35 PM

@puppyloverlauren

No problem. ^^ The graph should look like there's a line (or a different function, if you're doing that kind of math) and then a shaded part next to it. The line will be either dotted or straight.

The very first step is to find the equation of the line. You do that by first finding the slope. Pick two points on the graph to do this. Say the line has a point at (0,2) and (4,5). You use the equation for slope:

m = (y2-y1)/(x2-x1)
= (5-2)/(4-0)
= (3)/(4)
= 3/4

So the slope is 3/4. Great! Now, we find the full equation of the line by using the point slope form of a line and substituting one of the points we found above.
I'm going to pick the point (0,2).

y - y1 = m(x - x1)
y - 2 = 3/4(x - 0)
y - 2 = 3/4x
y = 3/4x + 2

So there's the equation of the line. c:

spiral • 3 October 2012 at 8:36 PM

Now, you're almost done. The last thing you need to do is see whether it's a less than, greater than, less than or equal to, or greater than or equal to graph. If the line is dotted, then it's either a less than or a greater than. If the line is solid, then it's either a less than or equal to or a greater than or equal to.

To determine this, pick one point from the shaded area. From our made up example, I'll say the point (3,4) is shaded, but not on the line. So we substitute this into our equation of the line:

4 ? 3/4 x 3 + 2
4 ? 9/4 + 2
4 ? 17/4
4 < 17/4

17/4 is greater than 4. As a result, we can determine that the y value is less than the x side of the equation. And you're done!

y < 3/4x +2 is our final answer.

silver_winged • 3 October 2012 at 8:38 PM

@puppyloverlauren

Wait a second . . . are you just working with a number line? Something like this: http://img.sparknotes.com/figures/5/50ca5e784bb7e4242910d5b8a571d103/number_line.gif ?

puppyloverlauren • 3 October 2012 at 8:41 PM

@silver_winged ah yes, sorry forgot what it was called ^^; I already got the answer for that one!

you can close this topic

sunshinecat • 3 October 2012 at 8:41 PM

@silver_winged I think so, because I've already posted and they said it wasn't what they wanted.

The way you do things is strange to me lol. Where are you from?

silver_winged • 3 October 2012 at 8:43 PM

@sunshinecat

:| I just spent a lot of time explaining that algebraic concept to someone who probably won't learn it for another few years.

I'm from America. c: Is New Zealand math that different?

sunshinecat • 3 October 2012 at 8:44 PM

@silver_winged It does seem quite different. I looked at a California algebra test paper and lots of it was quite unfamiliar.

silver_winged • 3 October 2012 at 8:45 PM

@sunshinecat

It would be interesting to compare the two. I'm taking calculus right now. ^^ Doing algebra again is kind of like walking when you're used to sprinting.