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 August 9th, 2008, 09:23 AM #1 Member   Joined: Aug 2007 Posts: 39 Thanks: 0 another straight line question the equations of two striaght lines are: ax + by + c = 0 dx + ey + f = 0 a. given that the lines do not intersect, write down an equation connecting a, b, d and e. b. given that the lines have the same y-intercept, write down another equation. c. given that the first line has identical x- and y-intercepts, write down a third equation. i have the answers, but i seriously have no flippin idea how the book got there. a. ae = bd b. ce = bf c. a = b could someone please explain how to get to these answers please? August 9th, 2008, 11:09 AM #2 Senior Member   Joined: Oct 2007 From: Chicago Posts: 1,701 Thanks: 3 Re: another straight line question a) Since the two lines don't intersect, it means the slope of the two lines is the same. I.e. a/b = d/e. Cross multiplying gives us ae=bd. b) They have the same y-intercept and have the same slope. This means they are the same line. This makes us want to say that c=f. But the line x+y+1=0 is the same as the line 2x+2y+2=0. We arrive again at the two equations having similar ratios. So b/c = e/f, or bf=ce (also, af=cd works) c)look at the equation in point-slope form: y=-ax/b -c/b Now if a =b, we have y=-x -c. Which says it has a slope of -1. This means that the x-intercept and y-intercept are identical. If you're not convinced y=-x-c x+y=-c x=-y-c. Letting y = 0, (so we have the x-intercept) x=-c. Which is also the y-intercept. If you need me to be more specific, let me know. Cheers, Cory August 9th, 2008, 11:15 AM #3 Member   Joined: Aug 2007 Posts: 39 Thanks: 0 Re: another straight line question wow thanks! your a real genius!  August 10th, 2008, 07:45 AM   #4
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Re: another straight line question

Quote:
 Originally Posted by Kiranpreet wow thanks! your a real genius! I wish. I've just been doing this long enough... Tags line, question, straight 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 OriaG Calculus 3 September 4th, 2012 01:43 AM shalikadm Algebra 3 January 24th, 2012 10:48 PM poochie03 Algebra 3 November 5th, 2011 12:00 PM outsos Algebra 7 December 23rd, 2010 07:15 PM Kiranpreet Algebra 2 August 9th, 2008 09:29 AM

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