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Thread: Help with finding y-intercept using gradient and x -intercept

  1. #1
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    Help with finding y-intercept using gradient and x -intercept

    Given gradient (-2/3) y intercept (3)
    Question Use the gradient to find the y coordinate on the line were
    x=6
    my working out
    y2-y1/x2-x1
    y-0/0-6 = 2/3 (Negative)
    y/-6=2/3(Negative)
    y=(negative) 2/3 x (negative) 6/1
    y=12/3
    y=4
    I know the answer is supposed to be negative but I don't know where I went wrong.
    Also the answer is negative 1.
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  2. #2
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    Re: Help with finding y-intercept using gradient and x -intercept

    Quote Originally Posted by Ito4 View Post
    Given gradient (-2/3) y intercept (3)
    Question Use the gradient to find the y coordinate on the line were
    x=6
    my working out
    y2-y1/x2-x1
    y-0/0-6 = 2/3 (Negative)
    y/-6=2/3(Negative)
    y=(negative) 2/3 x (negative) 6/1
    y=12/3
    y=4
    I know the answer is supposed to be negative but I don't know where I went wrong.
    Also the answer is negative 1.
    Doing it your way using the gradient formula.

    You have the point (0, 3) ie the y intercept. And you need to find y where (6, y) is on the line.
    So equating gradients:

     \frac{y-3}{6-0}=\frac{-2}{3}
    y-3=\frac{-2}{3}*6
    y-3=-4
    y=-1

    Alternatively:
    The equation of a straight line with gradient m and y-int c is y=mx+c.

    So
    the equation of the line with gradient \frac{-2}{3} and y-int 3 is  y=\frac{-2}{3}x+3

    Now substitute x=6 to get y=-1
    Last edited by Debsta; Jan 12th 2017 at 03:31 AM.
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  3. #3
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    Re: Help with finding y-intercept using gradient and x -intercept

    Quote Originally Posted by Ito4 View Post
    Given gradient (-2/3) y intercept (3)
    Question Use the gradient to find the y coordinate on the line were
    x=6
    my working out
    y2-y1/x2-x1
    y-0/0-6 = 2/3 (Negative)
    You sad "y intercept 3" but there is no "3" in this equation! You are using (0, 0). That would be with the line crossing the y-axis at the origin, not at (0, 3). Also in the numerator, your "y2" is "y" and "y1" is 0 but the denominator has "x2" equal to 0 and "x1" equal to 6. That's reversed. Your given point is (x2, y2)= (6, y) and the y intercept is (x1, y1)= (0, 3). It should be "y- 3" in the numerator and "6- 0" in the denominator:
    \frac{y- 3}{6- 0}= -\frac{2}{3}so that y- 3= -\frac{2}{3}(6)= -4 and then y= 3- 4= -1.

    y/-6=2/3(Negative)
    y=(negative) 2/3 x (negative) 6/1
    y=12/3
    y=4
    I know the answer is supposed to be negative but I don't know where I went wrong.
    Also the answer is negative 1.
    Why do you keep writing the word "negative"? Doesn't your key board have a "-" just to the right of "0"?
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