Help W. Graphing Equations.

Help W. Graphing Equations. solving for y?

#1 bohemianheart.

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Posted 30 September 2007 - 12:38 PM

Hello well i'm not sure if i'm in the right place or not. But um... I have a math worksheet. & they want us to graph equations like ex: y=3/2X-1 (but i already know how to do that).

There are these that i'm having trouble w.:

-2X+3Y+-9 , i'm not sure what to do w. this. Do i solve for Y? When i did to solve for Y, i get Y=-3+2X. Am i wrong?

3X+Y=0 , & for this one I got Y=0-3X

-3X+4Y=8 , as for this one i got Y=4-3X

X+2Y+6=0 , for this one i got Y=3-X

7X-5Y=20 , this one is the most confusing for me. I'm not sure if i divide the -5Y to the 20 or to both 20 or 7. ): but for this one i got Y=-4-7X

-8X-3Y=12 , i got Y=-6+8X

As for these 20X+80Y=O & 30X+50Y-100=0 i'm not sure how to do these, because their very very confusing. I'm not positive if i did these correctly because when i graph these we have to find the intersection between two equations. It's one of those 'What did the teacher do w. ogar's cheese report?' those find the word worksheets. ): Please help me. I'm sorry that my post is very long. T_T

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#2 joonieballoonie

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Posted 30 September 2007 - 01:10 PM

QUOTE(innocentlove @ Sep 30 2007, 01:38 PM) <{POST_SNAPBACK}>

Well, there are a bunch of ways you can graph equations - since these are just linear, the easiest method might just be finding the x and y-intercepts and connecting them.

1.) So...for -2X+3Y+-9, you can make it -2x+3y=9, and just find the intercepts by substituting 0 for x and y. X-intercept would be -9/2 and y-intercept would be 3. So your points are (-9/2, 0) and (0,3) and just connect the points. The equation you got was wrong - if you solve for y, you'd get y=-2/3x+3.

2.) 3X+Y=0
You get y=-3x, so that means y-intercept is 0, and just do rise/run. If you go to the right one and down three, you get the point (1,-3). Connect (0,0) and (1,-3), and that's your line.

3.) -3X+4Y=8
Substitute 0, and you'll get (-8/3,0) for your x-intercept and (0,2) for your y-intercept. The equation would be y=3/4x+2.

4.) X+2Y+6=0
Substitute 0, and you'll get (-6,0) for your x-intercept and (0,-3) for your y-intercept. The equation would be y=-1/2x-3.

5.) 7X-5Y=20
You said you had trouble with this, so I'll just go through the whole thing step-by-step.

7x-5y=20
7x-7x-5y=20-7x
-5y=-7x+20
-5y/-5=(-7x+20)/-5
y=7/5x-4

Or you can get the x and y-intercepts:

X-intercept:
7x-5(0)=20
7x=20
x=20/7
(20/7,0)

Y-intercept:
7(0)-5y=20
-5y=20
y=-4
(0,-4)

6.) -8X-3Y=12
Substitute 0, and you'll get (-3/2,0) for the x-intercept and (0,-4) for the y-intercept. The equation would be y=-8/3x-4.

I think you might have the idea of solving for y wrong - you have isolate y by moving x to the other side and everything by the coefficient of y. It seems like you only divide the constant by the coefficient of y, but you have to divide the coefficient of x by the coefficient of y, too.

Hope it helped.