Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Please double check my work

  1. Apr 6, 2004 #1
    Please double check my work !!!

    Hi, I basically did out all the poroblems. I just need someone to double check whether I made any mistakes in the answers or procedures. (My prof is a real dickhead and marks down for the smallest mistake) Also can I simplify any answers, just like obvious stuff?

    Thanks for the help!!!





    1)

    f(x) = x / (400-x)

    f'(x) = (400-x)(1) - (-1)(x) / (400-x)^2
    f'(x) = (400-x+x) / (400-x)^2
    f'(x) = 400 / (400-x)^2

    2)

    g(x) = (2x+3)/(x-5)

    g'(x) = (x-5)(2)-(1)(2x+3) / (x-5)^2
    g'(x) = (2x-10-2x-3) / (x-5)^2
    g'(x) = -13 / (x-5)^2

    3)

    y = (3x-4)/(x^3+1)

    y' = (x^3+1)(3x)-(3x^2)(3x-4) / (x^3+1)^2
    y' = [(3x^4+3x)-9x^3+12x^2] / (x^3+1)^2
    y' = (3x^4-9x^3+12x^2+3x) / (x^3+1)^2

    4)

    f(x) = (3x^2+2x) / x^5

    f'(x) = (x^5)(6x+2)-(5x^4)(3x^2+2x) / (x^5)^2
    f'(x) = (6x^6+2x^5-15x^6-10x^5) / (x^5)^2
    f'(x) = (-9x^6-8x^5) / (x^5)^2

    5)

    h(x) = sqrt(3+2x)

    h(x) = (3+2x)^(1/2)
    h'(x) = 1/2(3+2x)^(-1/2)(2)

    6)

    y = (x^4-4x^2+x)^-5

    y' = -5(x^4-4x^2+x)^-6(4x^3-8x+1)

    7)

    y = (x)sqrt(3x+4)

    y' = (x)1/2(3x+4)^(-1/2)(3)(sqrt(3x+4))

    8)

    f(x) = x^3 + (100-x)^2

    f'(x) = 3x^2 + 2(100-x)(-1)

    9)

    y = 1 / (x+2)^2

    y = (1)(x+2)^-2
    y'= -2(x+2)^-3(1)
    y'= 1/(-2(x+2)^3)

    10)

    h(x) = 1 + sqrt(x) / (x^5+3)

    h'(x) = (x^5+3)(1+1/2(x)^(-1/2))-5x^4(1 + sqrt(x)) / (x^5+3)^2

    11)

    g(x) = (x-4)^8 * (x+3)^9

    g'(x) = (x-4)^8 * 9(x+3)^8 + 8(x-4)^7 * (x+3)^9

    12)

    f(x) = sqrt((3+x)/(2-x))

    f'(x) = 1/2(((3+x)/(2-x))^-1/2 * (((2-x) + (3+x)) / (2-x)^2)

    13) Find the 3rd derivative

    y = 8x^2 - 4x + 7

    3rd derivative does not exist because highest power is 2.

    14) Find the 3rd derivative

    f(x) = x^3 + 3/x
    f'(x) = 3x^2 + (-3)
    f'(x) = 6x

    f'(x) = 6
     
  2. jcsd
  3. Apr 6, 2004 #2
    13) Find the 3rd derivative

    y = 8x^2 - 4x + 7

    3rd derivative does not exist because highest power is 2.


    third derivative does exist - it's just 0
    y'=16x-4
    y''=16
    y'''=0
     
  4. Apr 6, 2004 #3
    Physiczman,

    your answer to q14 contains a mistake.





    f(x)=x^3 + 3/x
    f'(x)=3x^2-3/(x^2)
    f''(x)=6x + 6/(x^3)
    f'''(x)=6 - 18/(x^4)

    This because 3/x = 3x^(-1).
     
  5. Apr 7, 2004 #4
    thansk guys.


    Yea i tend to leave out the part where u square the denominator in the quotient rule. Thanks for the head up!
     
  6. Apr 7, 2004 #5
    for #3) Can I do

    y' = (3x^4-9x^3+12x^2+3x) / (x^3+1)^2
    y' = 3x(x^3-3x^2+4x+1) / (x^3+1)^2

    ?


    for #5) Can i do

    h'(x) = 1/2(3+2x)^(-1/2)(2)
    h'(x) = (3+2x)^(-1/2)

    ?

    Im not sure about 10 and 12 it seems like something could be done or?


    TRhanks again guys i really appreciate it
     
  7. Apr 7, 2004 #6

    matt grime

    User Avatar
    Science Advisor
    Homework Helper

    I'm sure your professor respects you just as much. (A mistake is a mistake is a mistake.) Sorry that you think you should get better marks even though you got it wrong; let's hope he (or she) doesn't make any silly mistakes when assigning your grade at the end of the course.
     
  8. Apr 7, 2004 #7
    well i think i was exaggerating a bit when I said he was a dickhead. Hes a great guy but very picky. :cool:

    When did I say or even imply that I want better marks even though Im getting it wrong?
     
    Last edited: Apr 7, 2004
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Please double check my work
  1. Please help check work (Replies: 1)

Loading...