My Math Forum Help me figure out this graphing point please

 Algebra Pre-Algebra and Basic Algebra Math Forum

 September 19th, 2014, 03:22 AM #1 Member   Joined: Sep 2014 From: Kansas Posts: 34 Thanks: 1 Hi. I have to graph the following on an online graph. The graph only allows the points to be split in half. In other words, 10, 10.5,11,11.5 etc. The x intercept is 18.18...... So there is no way to graph it using that. So they did something I haven't seen before in the text, nor did they explain themselves. Here is the equation: 5.5x+12.5y=100. So the y int is (0,8). So in order for them to be able to graph this they substituted x with 12.5 and solved for y coming up with (12.5,2.5). This will fit on the graph and I can see when they connect the plots with a line it does appear to have the x-int slightly past 18 so I am sure that part is right. At any rate I don't understand the whole idea of plugging in 12.5 for x or how it works etc. Could you shed some light on this please? Thanks! I don't have class till Monday and don't want to have to think about this any longer than I have to lol. Last edited by skipjack; September 19th, 2014 at 04:48 AM.
 September 19th, 2014, 04:01 AM #2 Newbie   Joined: Sep 2014 From: . Posts: 15 Thanks: 2 I'm not sure what your question is, but I'll try to answer. $$5.5x+12.5y=100$$ Explicit form of the equation: $$y=-\frac{5.5}{12.5}x+8$$ Plugging in $x=0$, our line will cross y-axis in $8$, therefore our first point is $(0,8 )$, and to get the other point (because we need 2 points to draw a straight line), our best choice is if we substitute $x$ with $12.5$ because then they will cancel out: $$y=-\frac{5.5}{\cancel{12.5}}\cancel{12.5}+8$$ leaving us with $$y=-5.5+8=2.5$$therefore our other point is $(12.5, 2.5)$ So you should always turn the equation into explicit form, which means that you make y on left side and everything else on the right side. That alien face is a smiley that you get when you type '8' and ')' together. Last edited by skipjack; September 19th, 2014 at 04:46 AM.
 September 19th, 2014, 05:00 AM #3 Senior Member   Joined: Apr 2014 From: Glasgow Posts: 2,157 Thanks: 732 Math Focus: Physics, mathematical modelling, numerical and computational solutions It seems to me like you should all use a different plotting tool!
 September 21st, 2014, 04:02 PM #4 Member   Joined: Sep 2014 From: Kansas Posts: 34 Thanks: 1 Thanks! You are 'both' correct. This is just one of those things I seen it right off when I logged on here. It's just threw me for a leep the way they presented it. I came up with the correct x-inter of 18 something or other and I just got brain frozen on that I guess. Thanks again! Last edited by skipjack; September 22nd, 2014 at 10:49 PM.
 September 21st, 2014, 04:11 PM #5 Member   Joined: Sep 2014 From: Kansas Posts: 34 Thanks: 1 Ok wait a minute, so this is not so clear to me after all. I haven't seen one like this yet and I don't understand the substitution thing. I always thought that what you did to one side of the equation you had to do to the other, including taking the 8*12.5. So apparently this is something that hasn't been covered because I don't get it. Yet anyhow lol. I guess I'm somewhat starting to understand this. I'll come back and look at it again later. Last edited by skipjack; September 22nd, 2014 at 10:50 PM.
 September 22nd, 2014, 04:46 PM #6 Member   Joined: Sep 2014 From: Kansas Posts: 34 Thanks: 1 Ok coming back to this after I break I understand. Not that difficult just different than what I'm used to. Thanks again. Last edited by skipjack; September 22nd, 2014 at 10:51 PM.

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