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Math Help - Help please how to do this?

  1. #1
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    Help please how to do this?

    f(x) = ax˛ + bx + c. If f(1) = 6, f (-1) = 8, f(-2) = 18, find f (2)
    Can someone please do the solution for me. I know that the answer is 14 but I dont know how to get that.
    Thanks!
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  2. #2
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    e^(i*pi)'s Avatar
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    Quote Originally Posted by Detanon View Post
    f(x) = ax˛ + bx + c. If f(1) = 6, f (-1) = 8, f(-2) = 18, find f (2)
    Can someone please do the solution for me. I know that the answer is 14 but I dont know how to get that.
    Thanks!

    f(1) = a + b + c =6
    f(-1) = a -b + c = 8
    f(-2) = 4a-2b+c = 18

    You have three equations and three variables
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  3. #3
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    Wait so if i put
    f(2) = 4a + 2b + c
    then what do I do?
    Is that the right method?
    Thanks
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  4. #4
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    Quote Originally Posted by Detanon View Post
    Wait so if i put
    f(2) = 4a + 2b + c
    then what do I do?
    Is that the right method?
    Thanks
    You're meant to get the values of a, b and c first by solving the equations given to you in post #2.
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  5. #5
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    Quote Originally Posted by Detanon View Post
    f(x) = ax˛ + bx + c. If f(1) = 6, f (-1) = 8, f(-2) = 18, find f (2)
    Can someone please do the solution for me. I know that the answer is 14 but I dont know how to get that.
    Thanks!
    the first time, you need find a, b, c:
    we have f(1)so x=1 and change into the f(x)=ax^2+bx+c
    then f(-1) and f(-2) is same the f(1)

    so we have

    a+b+c=6
    a-b+c=8
    4a-2b+c=18
    you can find a, b,c
    then change a, b, c you found go into f(x)=ax^2+bx+c
    finally f(2) you can find easily.
    if you didnot understand you can ask me and i can explain again. sorry my english bad something i wrote wrong .
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  6. #6
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    e^(i*pi)'s Avatar
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    You can find b by: f(1)-f(-1)
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  7. #7
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    Quote Originally Posted by e^(i*pi) View Post
    You can find b by: f(1)-f(-1)
    correct i did not remember this. thanks khakha
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  8. #8
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    Thanks, yo've helped me alot.

    Quote Originally Posted by e^(i*pi) View Post
    You can find b by: f(1)-f(-1)
    wait, so does that mean
    6 - 8 then.
    so b is -2?
    Last edited by mr fantastic; December 8th 2009 at 03:13 PM. Reason: Merged posts (and part of post moved to a new thread).
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  9. #9
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    e^(i*pi)'s Avatar
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    Quote Originally Posted by Detanon View Post
    wait, so does that mean
    6 - 8 then.
    so b is -2?
    No, look again and remember that if we take away a negative we must add it.

    ie b-(-b) = 2b
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  10. #10
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    Quote Originally Posted by Detanon View Post
    wait, so does that mean
    6 - 8 then.
    so b is -2?
    f(1)-f(-1)=(a+b+c)-(a-b+c)=6-8
    a+b+c-a+b-c=-2
    2b=-2
    b=-1
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