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

Functions problems help?

  1. Feb 22, 2010 #1
    1.A Bicycles manufacturer estimates that it can price its bicycles at P=140-0.02x dollars each.
    where x is number sold. the cost of producing x bicycles is 900-0.01x^2 dollars. Determine the marginal profit when 20 bicycles are made.

    Answer is $59.60


    2.A rocket is launched straight up. there is an observation stations 7miles from the launching site.a what rate distance between the rocket and station increasing when when rocket is 24 miles high and traveling at 200 miles per hour?

    answer is 1920 miles per hour.

    Note: it is not homework problems!!
    just my book exercise:

    I try is hardly but i cant solve please help me.

    thanks a lot

    I following all this but answer coming wrong

    revenue = (number sold)(price)

    profit = revenue - cost

    marginal profit is the derivative of the profit function.

    2. let y = height of the rocket , r = distance between the rocket and the station

    y^2 + 7^2 = r^2
    take the derivative of the above equation w/r to time , then sub in your given/calculated values to determine frac{dr}{dt}

    y = 24 miles and \frac{dy}{dt} = 200 mph
  2. jcsd
  3. Feb 22, 2010 #2


    User Avatar
    Science Advisor

    If you sell [itex]P=140-0.02x[/itex] bicycles at x dollars each, you will bring in [itex](140- 0.002x)x= 140x- 0.02x^2[/itex] dollars in revenue. Since the cost of those bicycles is [itex]900-0.01x^2[/itex] your profit will be [itex]140x- 0.02x^2- 900+ .01x^2= -900+ 140x- .01x^2[/itex]. Evaluate the derivative of that when x= 20.

    Is the problem that you can't take the derivative? Both of these use the simple [itex](x^n)'= n x^{n-1}[/itex].
    Here, 2y dy/dt= 2r dr/dt. You know y= 24, ([itex]r= \sqrt{24^2+ 49^2}[/itex], and dy/dt= 200 so put those numbers in and solve for dr/dt.
  4. Feb 26, 2010 #3

    for first one answer profit derivative is 140 -0.02x and the i put x=20 it is become 139.6
    but answer is 59.60
  5. Feb 26, 2010 #4

    if i remove 2 on both sides


    boths answer are coming wrong . please help me
  6. Feb 26, 2010 #5


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    Something doesn't seem right here. Cost of production should increase with x, but the expression actually decreases. Are you sure you copies the expressions correctly?
  7. Feb 28, 2010 #6

    yes sir i copy correct .
    but both answer coming wrong (not only one)
  8. Feb 28, 2010 #7


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    I agree with your answer of $139.6 for the bicycle profit problem. Perhaps the book answer is wrong, it does happen sometimes.
    The 54.56 is wrong. This number should be the distance from the station to the rocket, when the rocket is 24 miles above the launch site ... just use the pythagorean theorem for that.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook