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Math Help - Transformations

  1. #1
    Newbie
    Joined
    Jan 2009
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    1

    Transformations

    How would you solve this question:

    The graph of y=f(x) is expanded vertically about the line y=k by a factor of 5. The point (-3,2) on the graph of y=f(x) has its image under the transformation at the point (-3,22). Find the value of k.

    The only way I know how to solve this is with guess and check. >_<
    Is there a faster and easier way to solve it?
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  2. #2
    Junior Member
    Joined
    Nov 2008
    Posts
    61
    Well I don't know about faster, but definitely more elegant. This expansion will cause all points to be five times further away from y=k. So, a point was d away from k, then after expansion it will be 5d away from k. So let's take the point they give us, (-3,2) goes to (-3,22).

    Now since 2-->22 (ie. it increases), then 2>k. Otherwise 2-->a smaller number. So the distance from (-3,2) to y=k is 2-k. Likewise, the distance from (-3,22) to y=k is 22-k. This must be 5 times 2-k so:

    22-k=5(2-k)
    22-k=10-5k
    4k=-12
    k=-3
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