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Math Help - Urgent need of help with finding radius

  1. #1
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    Urgent need of help with finding radius

    A central angle of 60degrees subtends an arc of 18pi inches. Find the radius of the circle.

    do you use this: S=2*pi*r?

    If so, how do you use it? I know that when r = 1, the central angle = the arc. However, you need to find the radius in this problem. So what do I do?
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  2. #2
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    Quote Originally Posted by lax600 View Post
    A central angle of 60degrees subtends an arc of 18pi inches. Find the radius of the circle.

    do you use this: S=2*pi*r?

    If so, how do you use it? I know that when r = 1, the central angle = the arc. However, you need to find the radius in this problem. So what do I do?
    Change your angle into radians (it's the simplest way) bearing in mind that 180^o = \pi \text{rad}

    Then use r = \frac{l}{\theta}

    Spoiler:
    r = \frac{18\pi}{\frac{\pi}{3}} = 18\pi \times \frac{3}{\pi} = 54 \text{inches}
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  3. #3
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    Quote Originally Posted by lax600 View Post
    A central angle of 60degrees subtends an arc of 18pi inches. Find the radius of the circle.

    do you use this: S=2*pi*r?

    If so, how do you use it? I know that when r = 1, the central angle = the arc. However, you need to find the radius in this problem. So what do I do?
    s = r\theta , where \theta is the central angle in radians
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  4. #4
    Senior Member pacman's Avatar
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    "A central angle of 60degrees subtends an arc of 18pi inches. Find the radius of the circle. do you use this: S=2*pi*r? If so, how do you use it? I know that when r = 1, the central angle = the arc. However, you need to find the radius in this problem. So what do I do?"

    theta = 60 degrees = pi/3

    S = 18 pi

    if S = r(theta)

    r = S/theta = (18 pi)/(pi/3) =18(3) =
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