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*I think that the first two use F.O.I.L but i still cant get it

  • Math -

    I'm afraid I do not catch your question.

    Are you supposed to find f(4b) when f(x)=4x^2+5x-5? Or are you supposed to find f(x) when f(4b)=4x^2+5x-5. The second option does not make sense because the expression f(4b) should not leave any trace of "x" when x=4b.
    Please clarify.

  • Math -

    x=4b sorry

  • Math -

    Would the question then be:
    If f(x)=4x^2+5x-5, find f(4b).
    In this case,
    f(4b)= 4(4b)² + 5(4b) -5
    = 4(16b²) + 20b -5
    = 64b² + 20b -5

  • Math -

    Okay thank you that really makes more since your a lifesaver! Do you know how to do number 2 and 3 that you could walk me through too? I have literally tried them 11 times and it still says they are wrong

  • Math -

    First step: write down the function in terms of x:

    Then substitute x by what is in the parentheses (x+1) in this case. Don't forget the parentheses everywhere you make a substitution:
    f(x+1)=8(x+1)² + (x+1) - 3
    Expand and collect terms as required:
    =8(x+1)² + (x+1) - 3
    =8(x²+2x+1) + x+1 -3
    =8x² + 16x + 8 +x+1 -3
    =8x² + 17x + 6

    Proceed the same way for number 3. Post for checking if necessary. The answer is 8x²-x-3.

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