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Math Help - Algebra 2 - write the equation of the parabola in vertex form? Help!

  1. #1
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    Exclamation Algebra 2 - write the equation of the parabola in vertex form? Help!

    Hi I'm new to this forum! I'm in Algebra 2 and I have a midterm to take tomorrow and I really need help with this problem!

    It says:

    "Suppose a parabola has vertex (-8, -7) and also passes through the point (-7, -4). Write the equation of the parabola in vertex form."

    and the choices are:

    a. y = (x+8)^2 - 7
    b. y = 3(x-8)^2 -7
    c. y = 3(x+8)^2 - 7
    d. y = 3(x+8)^2 +7

    Also, if you could just tell me how you got the answer, that'd be great. I'm just looking over my past tests because my math teacher said that's what the midterm will test us on, and I got this problem wrong. So the answer isn't b. Could anyone explain why?

    Thanks,
    Brooke
    Last edited by wakefuldoze; January 2nd 2013 at 10:55 AM.
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  2. #2
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    Re: Algebra 2 - write the equation of the parabola in vertex form? Help!

    Substitute both points and see into which equation they fit.
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    Re: Algebra 2 - write the equation of the parabola in vertex form? Help!

    emakarov - Is it choice a?
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  4. #4
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    Re: Algebra 2 - write the equation of the parabola in vertex form? Help!

    Substituting (-7, -4) into (a) gives -4 = (-7 + 8)^2 - 7 = 1 - 7 = -6. (A buzzer sound.)
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    Re: Algebra 2 - write the equation of the parabola in vertex form? Help!

    Uh, ok? I just need to know how to get to the correct answer. I really struggle with math and I'm running short on time and I would love to just move on. Could anyone explain it to me? Thanks
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  6. #6
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    Re: Algebra 2 - write the equation of the parabola in vertex form? Help!

    Don't be lazy. Just substitute the numbers into the equations and calculate.
    Thanks from earboth
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    Re: Algebra 2 - write the equation of the parabola in vertex form? Help!

    Quote Originally Posted by wakefuldoze View Post
    ...
    It says:

    "Suppose a parabola has vertex (-8, -7) and also passes through the point (-7, -4). Write the equation of the parabola in vertex form."

    and the choices are:

    a. y = (x+8)^2 - 7
    b. y = 3(x-8)^2 -7
    c. y = 3(x+8)^2 - 7
    d. y = 3(x+8)^2 +7

    ...
    Quote Originally Posted by wakefuldoze View Post
    Uh, ok? I just need to know how to get to the correct answer. ...
    1. The equation of a parabola with the vertex V(x_V, y_V) and the coefficient a is:

    y = a(x-x_V)^2+y_V

    2. Replace the coordinates of the vertex and you'll get:

    y = a(x+8)^2-7

    3. The coordinates of the given point have to satisfy this equation:

    -4=a(-7+8)^2-7

    Solve for a.
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  8. #8
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    Re: Algebra 2 - write the equation of the parabola in vertex form? Help!

    Thanks earboth!

    Also, for this other problem:
    "What steps transform the graph of y = x^2 to y = -(x+3)^2 + 5"

    The choices are:

    a. translate 3 units to the right, translate down 5 units
    c. reflect across the x-axis, translate 3 units to the left, translate up 5 units
    d. reflect across the x-axis, translate 3 units to the right, translate down 5 units

    When is do you reflect across the x-axis? And I'm guessing that it's either a or d? Any help? Thank you to anyone who helps me!
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  9. #9
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    Re: Algebra 2 - write the equation of the parabola in vertex form? Help!

    Wait no, I bet it's c. Is it?
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  10. #10
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    Re: Algebra 2 - write the equation of the parabola in vertex form? Help!

    Yes, it's c.
    Thanks from wakefuldoze
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