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How is the circumference of a circle with radius 9cm related to the circumference of a circle with diameter 9cm?
i don't understand this.Help?? Thanks:)

  • math -

    Start with the equation of circumference of a circle:

    c = pi x d.

    The diamter is 2 x radius. So, the diameter of the first cirlce is 18cm. You know the diameter of the second circle. So, how do the circumferences compare?

  • math -

    is the second circle twice as big? like is that how they compare?

  • math -

    the circumference of the larger circle is indeed twice as long as that of the smaller one.

    but...
    the area of the larger circle is 4 times that of the area of the smaller.

    Proof:
    area of smaller = pi(4.5)^2 = 20.25pi
    area of larger = pi(9^2) = 81pi

    20.25pi/(81pi) = 1/4

  • math -

    alright thx!

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