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Homework Help: What is the radius of the circle?

  1. Oct 5, 2005 #1
    "Use the formula s = rθ ( "θ" is NOT an 8) to solve the following problem. An arc of 3 feet subtends a central angle of 1.8 radians. What is the radius of the circle?"

    How do I do this problem?
  2. jcsd
  3. Oct 5, 2005 #2


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    It might help to know what "the formula s = rθ" means.
    (arc length)=(radius)(central angle subtended).

    Could you do the problem if the angle is [tex]2\pi=2(3.1415...)[/tex] radians?
  4. Oct 5, 2005 #3
    Nevermind...think I figured it out.

    Radius of 1.666666667 feet, or 20 inches sound right?
  5. Oct 5, 2005 #4
    Yes it does.

  6. Oct 5, 2005 #5


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    No it is not correct. Your given numbers have only 2 significant digits, how can you claim 10 digits? The correct answer should be 1.7 ft.

    Copying all the digits which show up on your calculator is a very bad habit. Learn to give answers which reflect the significant digits of the problem.
  7. Oct 5, 2005 #6
    Oh yeah, sorry I was talking about inches.
  8. Oct 6, 2005 #7


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    He said "20 inches" which is to two significant figures!

    GJBenn85: If you are taking calculus, you certainly should be able to solve and equation like s= rθ for r! I'm glad you wer able to figure it out.
  9. Oct 6, 2005 #8
    I know. It was a case of me trying to turn a simple problem into a more complex one. For some reason, I was thinking i had to convert the 1.8 radians into something else. Simple things smack me in the face. :)
  10. Oct 12, 2005 #9

    ... not to split hairs .. but ...
    20 inches has 1 significant figure
    20. inches has 2 significant figures

    PLUS .. many math classes don't require significant figures.. and if they do then the person has it so hammered into their head that they'd shudder at thinking about it .. AND .. writing all the digits from a calculator is a GOOD thing unless you're at a FINAL answer, and since there are most definitely 12 in / 1 ft, (a definition) ... then had the person done 1.7 ft it would be off and what if the measurements were given as exact values? ... values found in a theoretical sense of perfect measurements
  11. Oct 17, 2005 #10
    Sometimes it is according to the teachers. Mine say three sigfigs, unless it specifically says different.
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