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Quick maths check for algebra

  1. Sep 20, 2012 #1
    I would just like to someone to have a quick look at my maths for this question. I do not feel 100% that I am doing it right.

    Question: Two circles have a radii r cm and 4r cm.

    Find in terms of ∏ and r
    a) the area of the circle with radius 4r. Here are my working; by the way still cant get latex to work, I have tried to everything, could someone point me to a latex post to see where I am going wrong.

    area = ∏*r^2 ====> ∏*(4r)^2 =====> (16∏r^2)

    b) The area of the shaded ring:

    area = 16∏r^2 - ∏r^2 = (15∏r^2)

    c) The total length of the inner and outer edges of the shaded ring.

    Circumference = 2∏r =====> 2∏4r ======> 8∏r =====> 8∏r + 2∏r = (10∏r)

    The answers and the ones with brackets. I have attched a drawing of the image that goes with the question. Not sure if really needed.
     

    Attached Files:

  2. jcsd
  3. Sep 20, 2012 #2

    jedishrfu

    Staff: Mentor

    looks okay
     
  4. Sep 20, 2012 #3

    Mark44

    Staff: Mentor

    Your use of ====> and =====> above is pretty much meaningless. Your work would be improved if you described better what you are doing and used = to connect expressions with the same value.

    For example,
    Area of inner circle = ## \pi r^2##
    Area of outer circle = ## \pi (4r)^2 = 16\pi r^2##

    The right sides of the equations above are written using LaTeX. Right-click on either of them to see what I wrote.
    This would be better, IMO, as it gives the reader a better idea of what you are doing.
    Total circumference = Circumference of outer circle + Circumference of inner circle
    = ##2\pi (4r) + 2\pi r = 8\pi r + 2\pi r = 10\pi r##
     
  5. Sep 20, 2012 #4
    I have tried to put latex in, but have problems even when I write it in latex. I write the code, but it never changes to latex form. Do you have any suggestions why I have this problem?
     
  6. Sep 20, 2012 #5

    Mark44

    Staff: Mentor

    You have to put it inside a pair of [noparse][tex] and [/tex][/noparse] tags or a pair of [noparse][itex] and [/itex][/noparse]. Alternatively you can use a pair of $$ tags (at start and end) or a pair of ## tags (at start and end).

    Edit: It appears that [ noparse ] has stopped working. The first sentence above says

    You have to put it inside a pair of [ tex] and [ /tex] tags or a pair of [ itex] and [ /itex]. (Omit the leading spaces.)
     
    Last edited: Sep 21, 2012
  7. Sep 21, 2012 #6
    Thanks for the info, I was using [itex] as a tag to star and finish; maybe way it was not working.
     
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