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Math Help - Halp! Linear equation questions.

  1. #1
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    Sep 2008
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    Red face Halp! Linear equation questions.

    I'm supposed to find the y-intercept, slope, and equation for a couple problems. I'm having a lot of trouble with these. This first one has the two points on a graph (0,3) and (6,6) The y intercept is 3 and the slope is 3, but how do I find the equation? The second one has the points (2,18) and (6,6). the slope is -3 (right?) but how do you find the y-intercept when the x-axis skips 0? How do you find the equation for that? The third one has no y intercept because X stays the same. The points are (6,3) and (6,10). There is no slope and no y-intercept, so how do I say y goes up by 7 in an equation?
    Thank you SO much.
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  2. #2
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    A line's equation can be either y = mx + c or x = k.

    If x1 != x2 then m = (y2 - y1) / (x2 - x1) = slope and c = y-intercept
    when given two points on the line as (x1, y1) and (x2, y2).
    Substiture in first equation, y = 3x + 3 is the line equation.

    Similarly for the second, x1 = 2, y1 = 18, x2 = 6, y2 = 6 so
    m = (6 - 18) / (6 - 0) = -2
    so y = -2x + c, use any one of the points, say (0, 18) to put
    y = 18 = -2x + c = -2 * 0 + c = c
    equation is y = -2x + 18

    3rd equation: x1 = x2 so line equation is x = k, using any point, x = 6.
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  3. #3
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    Quote Originally Posted by hwhelper View Post
    3rd equation: x1 = x2 so line equation is x = k, using any point, x = 6.
    Thanks, but what should I say if I want to say y= something?
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