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 March 27th, 2014, 04:41 AM #1 Newbie   Joined: Mar 2014 Posts: 6 Thanks: 0 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 March 27th, 2014, 04:56 AM #2 Global Moderator   Joined: Dec 2006 Posts: 21,036 Thanks: 2274 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)"? March 27th, 2014, 05:00 AM #3 Newbie   Joined: Mar 2014 Posts: 6 Thanks: 0 Re: find the equations ah sorry mean B(5,-2) 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. March 27th, 2014, 05:36 AM #5 Newbie   Joined: Mar 2014 Posts: 6 Thanks: 0 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. March 27th, 2014, 07:27 AM #6 Newbie   Joined: Mar 2014 Posts: 6 Thanks: 0 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+ March 27th, 2014, 08:20 AM #7 Newbie   Joined: Mar 2014 Posts: 6 Thanks: 0 Re: find the equations edit i mean not 2/3 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 

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

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

You don't need to do it EXACTLY that way...but why take a chance...get my drift? March 27th, 2014, 11:33 AM #9 Newbie   Joined: Mar 2014 Posts: 6 Thanks: 0 Re: find the equations ok thanks Denis practise practise!!!! Tags equations, find Thread Tools Show Printable Version Email this Page Display Modes Linear Mode Switch to Hybrid Mode Switch to Threaded Mode Similar Threads Thread Thread Starter Forum Replies Last Post ilgaar Algebra 1 May 11th, 2013 09:41 AM kingkos Algebra 1 December 23rd, 2012 04:43 AM stevecowall Algebra 2 February 29th, 2012 05:38 AM carljohns_gta Algebra 7 July 30th, 2011 07:50 AM symmetry Algebra 1 September 10th, 2007 08:05 PM

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