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Homework Help: Need help with function problem

  1. Jul 13, 2005 #1
    If f(x) = 3x + 1
    and g(x) = (x^2 + 5x)^-1/2
    find g(f(x))
     
  2. jcsd
  3. Jul 13, 2005 #2
    Does this look right?

    f(x) = 3x + 1
    g(b) = (b^2 + 5b)^-1/2
    Let b=f(x)=3x+1
    Substitute 3x + 1 in for b:
    g(b)=((3x+1)^2+5* (3x+1))^-.5
    =(9x^2+6x+1+15x+5)^-.5
    =(9x^2+21x+6)^-.5
    g(f(x))= (9x^2+21x+6)^-.5
    Please advise
     
  4. Jul 13, 2005 #3

    HallsofIvy

    User Avatar
    Science Advisor

    Yes, that's exactly right. To find g(f(x)) you replace the "x" in the formula for g with the formula for f(x).
     
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