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March 27th, 2014, 04:41 AM   #1
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find the equations

Hello, hopefully I'm on the right bit

I need to find the equations of a few plots first is:

A(-1,-4) B(5-2)

I'd like to know if this is the correct way:

m= -2-4/5-1 = -6/4 =-3/2

so m= -3/2


-4=-3/2x + c

c=-11/2

so the equation is y=-3/2x-11/2
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March 27th, 2014, 04:56 AM   #2
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Take care to include all the necessary symbols. For example, did you mean "B(5-2)" to be "B(5,2)" or "B(5,-2)"?
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March 27th, 2014, 05:00 AM   #3
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Re: find the equations

ah sorry mean B(5,-2)
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March 27th, 2014, 05:27 AM   #4
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Re: find the equations

Quote:
Originally Posted by mintedm
Hello, hopefully I'm on the right bit

I need to find the equations of a few plots first is:

A(-1,-4) B(5-2)

I'd like to know if this is the correct way:

m= -2-4/5-1 = -6/4 =-3/2

so m= -3/2
No, it's not. The slope of the line through and is given by
or

That is what you are trying to do but you are being careless with signs!




I presume the "-4" is from (-1, -4) so x should be -1: -4= (1/3)(-1)+ c so c= -4+ 1/3= -12/3+ 1/3= -11/3

Quote:
c=-11/2

so the equation is y=-3/2x-11/2
No, it should be y= (1/3)x- 11/3. That, of course, is the same as 3y= x- 11 or x- 3y= 11.

Another way of doing this whole problem is to recognize that any (non-vertical) line can be written as y= mx+ c for some numbers m and c. Since the point (-1, -4) is on the line, -4= m(-1)+ c. Since (5, -2) is on the line, -2= m(5)+ c.

Eliminate c from the equations by subtracting the first equation from the second: (-2- (-4))= 5m+ c- (-m+ c) or (-2+ 4)= 2= 6m so m= 2/6= 1/3. You should see that the calculations there are exactly the same as before. Since m= 1/3, -2= (1/3)(5)+ c so c= -2- 5/3= -6/2- 5/3= -11/3 OR -4= (1/3)(-1)+ c so c= -4+ 1/3= -12/3+ 1/3= -11/3. In either case, y= (1/3)x- 11/3 again.
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March 27th, 2014, 05:36 AM   #5
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Re: find the equations

Explains a lot, thanks. Going to have another shot at it with your help, and see if I can get it correct this time before moving on.
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March 27th, 2014, 07:27 AM   #6
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Re: find the equations

ok next one P(1,3) Q(4,2)



= =

m=

3= (x)+c

3+() =

so the equation should be y=x+
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March 27th, 2014, 08:20 AM   #7
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Re: find the equations

edit i mean

not 2/3
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March 27th, 2014, 11:21 AM   #8
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Re: find the equations

Quote:
Originally Posted by mintedm
ok next one P(1,3) Q(4,2)
= =
m=
3= (x)+c
3+() =
so the equation should be y=x+
That's quite messy, Minter...if I was your teacher, I'd deduct points even if correct

y = mx + b [1]

m = (2 - 3) / (4 - 1) = -1/3
Substitute in [1]:
y = (-1/3)x + b [2]

Using P(1,3) and substituting x=1 and y=3 in [2]:
3 = (-1/3)(1) + b
3 = -1/3 + b
b = 3 + 1/3
b = 9/3 + 1/3
b = 10/3
Substitute in [2]:
y = (-1/3)x + 10/3

You don't need to do it EXACTLY that way...but why take a chance...get my drift?
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March 27th, 2014, 11:33 AM   #9
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Re: find the equations

ok thanks Denis practise practise!!!!
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