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C1 Question. Calculate y = mx + c, when m = infinity?? watch

    • Thread Starter

    Hi, I'm teaching myself C1, managing these questions fine but there's one style I'm having an up hill struggle with.

    Find the equations of the lines joining the following pairs of points. Leave your final answer without fraction in one of the froms y = mx + c OR ax + by + c = 0.


    (3, 7) and (3, 12).

    1st I calculate the gradient.
    12-7/3-3 = 5/0 = Infinity (straight line upwards)..

    So now I'll add (3, 7) into y = mx + c

    7 = 0 x 3 + c (I think this is where I'm making the mistake because m = infinity?)

    So if i continue doing it my way I get

    c = 7, therefore y = 3..

    The correct answer is x - 3 = 0 ??

    Can somebody shed some light on the matter for me?


    Infinity isn't a number. You've put m=0!
    It's best just to think of it geometrically, as you say it's a vertical line through x, hence the line is just x=3.

    If you knew that it was a straight line upwards, couldn't you have just gone straight to x=3, and then rearrange that to x-3=0

    (Original post by alex12alex11)
    If you knew that it was a straight line upwards, couldn't you have just gone straight to x=3, and then rearrange that to make y=x-3
    x=3 is not the same as y=x-3
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Updated: March 26, 2011

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