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

Assignment calculus,simultaneous equation and logarithm

  1. Jan 24, 2012 #1
    1. The problem statement, all variables and given/known data

    Hi, teachers

    I have been doing some work but I am confused.

    Have you ever heard of this great math website www.wolframalpha.com (Google for maths).

    This is one questions I done differantiate 3e^-4tsin(5t) and here is the link for the answers from


    Although what I did is confusing to what the answers there?

    This my solution

    y=3e^-4t sin(5t)

    u=3e^-4t v =sin(5t)

    dy/dt=-12e^-4t dv/dt=-5cos(5t)

    chain rule/product rule (1 of them)

    ds/dt= 3e^-4t x 5cos(5t) + sin(5t) x -12e^-4t


    =3e^-4t (5cos(5t) - 4sin (5t))

    Am I or the computer correct ?

    Also while you are there? May you look at these two questions below

    1) The table gives a firm’s monthly profits for their first few months of business. The numbers are rounded to the nearest £100.

    Number of months after start up (x) 1 2 3 4 5 6
    Profit for this month (£y) 500 800 1200 1900 3000 4800

    The firm’s profits, £y, for the xth month after start up are modelled by

    y = k x 10ax

    where a and k are constants.

    Plot this data on semi log paper to verify if this relationship is true.

    Its about plotting semi-log paper but am confused. Can you give me guide on where to start answering the question.

    Thanx agin

    Simultaneos question
    y = x2 – 5x + 5 a

    5x + 3y = 30. b

    Find location of points collide?

    Add a+b

    x2 + 2y -25=0

    quadratic formula use

    x= -2+ square root(2^2-(4x1x(-25))/2 x = 4.09901


    x= -2 - square root(2^2-(4x1x(-25))/2 x=-6.09

    subsitue y etc
  2. jcsd
  3. Jan 24, 2012 #2


    Staff: Mentor

    The first derivative is du/dt, not dy/dt.
    In the other derivative, you have a sign error.
    Why ds/dt? Your original function is y, so the derivative would be dy/dt.

    Also, don't use x for multiplication. If there happens to be a variable named x, you will confuse yourself.
    This looks fine to me.
    Please start a new thread with each of these problems.

    Be sure to include the three parts of the homework template.
    Again, don't use x for multiplication
  4. Jan 24, 2012 #3


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    Hello maali5. Welcome to PF !

    I used the link. What you asked WolframAlpha to differentiate, 3e^-4tsin(5t) , is equivalent to differentiating
    [itex]\displaystyle\frac{3 t\sin(5 t)}{e^4}\,.[/itex]​
    You need to put parentheses around the -4t if you want WolframAlpha to differentiate
    [itex]\displaystyle\frac{3 \sin(5 t)}{e^{4t}}\,.[/itex]​
    Write your function as 3e^(-4t)sin(5t) .
  5. Jan 25, 2012 #4

    Thanx for your help. I got it.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook