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Thread: Help with finding the x and y intercepts using the gradient

  1. #1
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    Help with finding the x and y intercepts using the gradient

    Given a gradient (3) and a x intercept (-3) supposed to find the y intercept.
    My working out
    Y/X = M
    Y/-3 =3
    Y/-3 X -3 = 3 x -3
    Y=-9
    But the book says it's 9, and I don't know why?

    Help with finding the x and y intercepts using the gradient-img_6495.jpg
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  2. #2
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    Re: Help with finding the x and y intercepts using the gradient

    The line crosses the x-axis at (-3, 0) and goes upward because it has positive slope- how could it possibly have negative y intercept?

    Your very first equation is wrong: Y/X= M is incorrect because that only applies when the graph passes through (0, 0). What is true is that \frac{\Delta y}{\Delta x}= M or \frac{y_2- y_1}{x_2- x_1}= M where (x_1, y_1) and (x_2, y_2) are two points on the line and \Delta y= y_2- y_1 and \Delta x= x_2- x_1. Here one point is (-3, 0) (the x-intercept) and another is (0, y) (the unknown y- intercept). You should have \frac{y- 0}{0- (-3)}= 3.
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  3. #3
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    Re: Help with finding the x and y intercepts using the gradient

    Thanks for answering but my question is how do you know it's going upward.
    Last edited by Ito4; Jan 11th 2017 at 03:43 AM.
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  4. #4
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    Re: Help with finding the x and y intercepts using the gradient

    No, these questions don't sound stupid- they sound like you are memorizing formulas without understanding what they mean! I know the line is going upward because that is what "slope" means! The slope measures the rate at which the y-value is increasing relative to the x-value. Positive slope means y is increasing, negative slope means y is decreasing.

    "for the very last equation y can't equal 9 because the answer will be 3."

    What answer? The answer to what question? The last equation is \frac{y}{3}= 3. y has to be 9 to satisfy that.
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