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B Circumference of a circle

  1. Sep 15, 2016 #1
    I just have a little question that i have read in the book of Thomas/Finney Calculus 9th edition that the circumference of a circle is 2pi,i can be wrong obviously but wasn't it supposed to be 2*pi*radius of the circle?
    Please help.
     
  2. jcsd
  3. Sep 15, 2016 #2

    berkeman

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    Staff: Mentor

    Both are correct if the radius r = 1. Can you scan the page in question and UPLOAD it?
     
  4. Sep 15, 2016 #3

    Math_QED

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    Homework Helper

    Most likely, they calculated the length of the curve ##y = \sqrt{1 - x^2}##. This is the unit circle with radius ##1## and thus the circumference is equal to ##2\pi##.

    If they would have calculated the length of the curve ##y = \sqrt{r^2 - x^2}##, the circumference would be equal to ##2\pi r##. This is the circle through the origin with radius ##r##.
     
  5. Sep 15, 2016 #4
    Oh, yes, thank you, they have said about the unit circle at first then said that it's circumference is 2pi. Sorry,I didn't notice . But thank you again, I was really confused.
     
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