Lines and t-tables
Date: 11/09/96 at 16:06:36 From: Emilee Anderson Subject: graphs -2x-4y= I have to do a t-table and I just don't get it at all!
Date: 01/27/97 at 10:19:16 From: Doctor Jimbo Subject: Re: graphs Emilee, People use t-tables to get an idea of what a graph looks like. Unfortunately, the equation you have submitted is missing a righthand side, so it's not an equation. (It's kind of like a sentence without a verb - it just doesn't make sense as it is.) I'll try changing your equation a bit, so that it makes sense. How about we try -2x-4y=3 instead? To use a t-table with this sort of an equation, some people make a list of one of the two variables and then figure out what the value of the other variable must be. For instance, we can make a table of x-values, and find out what y has to be for each of them. I'll try this table: x | y ------- 0 | 1 | 2 | 3 | Now, the task is to figure out what numbers have to go in the righthand side. Let's start with the first value for x, x=0. Now you ask yourself, "If x is 0, what does y have to be?" Plug in 0 for x, and our equation becomes: -2x-4y=3 (the original equation) -2(0) - 4y = 3 (since x = 0) 0 - 4y = 3 (-2*0 = 0) y = 3/(-4) (divide both sides by -4) y = -3/4 (simplify) Now I can put -3/4 in for y on the first line of my table. This means that based on our equation, if x is 0 then y has to be -3/4. Continuing on, I could use x = 1 from the next line, and find that -2(1) - 4y = 3 (x = 1) -2 - 4y = 3 (simplify) -4y = 5 (add 2 to both sides) y = -5/4 (divide by (-4) and simplify) When I'm done, my table will now look like this (I've left some for you to check): x | y --------- 0 | -3/4 1 | -5/4 2 | -7/4 3 | -9/4 To create a graph, you plot those points: (0,-3/4), (1,-5/4), (2,-7/4) and (3,-9/4). You should be able to connect them with a straight line. You may wonder how I chose the values for x. It doesn't really matter much. Just make sure you use a few, and that they're close enough to fit on a graph, but far enough apart to show you what's going on. One important thing about t-tables is that they work well with straight lines and fairly simple graphs. When you start working with really complicated equations, though, you have to be careful because t-tables don't always show you everything that's going on. Good luck. -Doctor Jimbo, The Math Forum Check out our web site! http://mathforum.org/dr.math/
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