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Thread: need help with simple function question

  1. #1
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    need help with simple function question

    if f'(x)=4x-3 and f(2)=7 find f(4)
    thank you

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    Re: need help with simple function question

    I am a year 11 student getting E's in EXT1 maths but I will give this question a go.
    f'(x)=4x-3
    f(2)=7
    find(4)
    we know that f(x)=2x^2 -3x
    because to find f' we times the power to the coefficient and reduce the power by 1.
    so lets test if f(2)=4
    4=2(2)^2-3(2)
    we get :
    4=2 which is wrong
    now we know that in quadratic equation there can be a constant which determines if vertex is down or up
    so lets make that constant 'c'
    so we get the equation
    7=2+c
    c=5
    now we know that the correct equation is f(x)= 2x^2-3x+5
    find f(4)
    let y be f(4)
    y=2(4)^2-3(4)+5
    =25
    Therefore f(4)=25
    parallaxd12 likes this.

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    Re: need help with simple function question

    Am I right? what does the textbook say?

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    Re: need help with simple function question

    Quote Originally Posted by RhythmKnight View Post
    Am I right? what does the textbook say?
    yes you are right the text book answer is 25, thank you very much

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    Re: need help with simple function question

    no worries.

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