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The area of circle c is 81 pi . if the radius of the circle is doubled, what is the circumference of the new circle ?

please answer and explain

  • math -

    162 pi

  • math -

    my answer is 162 pi too, but the answer is

    a. 18 pi
    b. 36 pi
    c. 36 pi^2
    d. 324 pi
    e. 324 pi^2

  • math -

    if a = 81pi, r = 9
    if r = 18, c = 36pi

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