Pre-Calculus Pre-Calculus Math Forum

October 18th, 2019, 11:21 PM   #1
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Equation of a Line passes through.

Hi all,
I’m taking a Pre-Cal and I have totally forgotten this Algebra stuff . Have looked everywhere, but still my work is wrong . The question ask to look for the perpendicular line (in Slope-Intercept) that pass through (1,-9) with the equation y=(4-x)/3. The correct answer is y=3x-12. Below is my work
Attached Images C5588421-9C3C-4F58-A8B3-807556A7D224.jpg (20.6 KB, 3 views) 7949D526-D917-4F71-BDCC-C5E280E19B4C.jpg (20.2 KB, 2 views) October 19th, 2019, 02:25 AM #2 Global Moderator   Joined: Dec 2006 Posts: 21,124 Thanks: 2332 You are supposed to find the equation of a line that passes through the point (1, -9) and is perpendicular to the line with equation y = (4 - x)/3. Perpendicular lines are parallel to the axes or have slopes whose product is -1. As the given line has slope -1/3, any line perpendicular to that line has slope 3. Hence the required line's equation, in point-slope form, is y = -9 + 3(x - 1). Writing that in slope-intercept form gives y = 3x - 12. Thanks from topsquark and idontknow October 21st, 2019, 01:23 AM #3 Newbie   Joined: Aug 2018 From: US Posts: 6 Thanks: 0 Why did you put “-9” beside “+3(x-1)” like that? I haven’t came across such equation y=-9+3(x-1) before. I tried intuitively instead with -9=3(x+1) but my answer was y=3x+21. Can you explain your method and why mine is wrong? Plz help, I got a test coming Last edited by skipjack; October 21st, 2019 at 02:46 AM. Reason: to change x+1 to x-1 in references to preceding post October 21st, 2019, 02:49 AM #4 Global Moderator   Joined: Dec 2006 Posts: 21,124 Thanks: 2332 As you haven't shown every step (including any assumptions and reasoning) you used, I can't tell where exactly you went wrong. If a line of known slope $m$ passes through a point $(x_1,\, y_1)$, its slope at any other point $(x,\, y)$ on the line is also $m$, so $\displaystyle m = \frac{y - y_1}{x - x_1}$. That implies $y = y_1 + m(x - x_1)$, which also holds for $(x,\, y) = (x_1,\, y_1)$ and is called the point-slope form of the line's equation. For further information, see this article. Thanks from topsquark Tags equation, line, passes 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 loksc Geometry 1 March 16th, 2017 01:56 PM GIjoefan1976 Algebra 2 April 13th, 2016 05:28 PM Zadek Algebra 1 November 17th, 2013 02:54 AM najaa Calculus 4 October 21st, 2012 09:20 AM shin777 Algebra 1 November 27th, 2011 08:21 PM

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