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Math Help - Simple Graphing Question

  1. #1
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    Simple Graphing Question

    Ok, this is going to sound stupid... because I am majoring in math... and have been graphing simple graphs for years...... but I was just tutoring a kid.. and was trying to figure out a quadratic equation from looking at the graph.... and I have a complete brain fade.

    So.... if the equation is y = (1/4)x^2
    I thought.... you would make a table multiply by the reciprocal..

    *4 | x | y|
    -8 | -2|4
    -4| -1| 1
    0 |0 |0
    4 |1 |1
    8 | 2 |4
    12|3 |9

    but to do it the other way... clearly when x=2.. y = (1/4)x^2 = 2*2*0.25 = 1
    .... but in the way high school tells you to do horizontal stretch... why is it saying when x is 4, y is 1?
    What am I missing?
    Last edited by mr fantastic; December 8th 2009 at 11:55 AM. Reason: rt --> de
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  2. #2
    MHF Contributor
    Grandad's Avatar
    Joined
    Dec 2008
    From
    South Coast of England
    Posts
    2,570
    Hello gwalker11
    Quote Originally Posted by gwalker11 View Post
    Ok, this is going to sound stupid... because I am majoring in math... and have been graphing simple graphs for years...... but I was just tutoring a kid.. and was trying to figure out a quadratic equation from looking at the graph.... and I have a complete brain fade.

    So.... if the equation is y = (1/4)x^2
    I thought.... you would make a table multiply by the reciprocal..

    *4 | x | y|
    -8 | -2|4
    -4| -1| 1
    0 |0 |0
    4 |1 |1
    8 | 2 |4
    12|3 |9

    but to do it the other way... clearly when x=2.. y = (1/4)x^2 = 2*2*0.25 = 1
    .... but in the way high school tells you to do horizontal stretch... why is it saying when x is 4, y is 1?
    What am I missing?
    I'm not at all sure why you would want to multiply by the reciprocal. This is a method that I have never seen! But if you do it this way, you must first re-write the equation as:

    y=(\tfrac{1}{2}x)^2

    So you'll need to multiply by 2, not 4. Then start with multiples of 2:

    E.g.
    (-4)\text{ = value of }x \rightarrow (-2) \rightarrow 4 = \text{ value of }y

    (-2)\text{ = value of }x\rightarrow (-1)  \rightarrow 1 = \text{ value of }y

    ... and so on.

    Does that make sense?

    Grandad
    Last edited by mr fantastic; December 8th 2009 at 11:56 AM.
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