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Math Help - ...finding the radius

  1. #1
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    ...finding the radius

    The points (8, 4) and (2, 2) are the ends of a diameter of a circle. Find the coordinates of the centre and the radius.


    I know the centre is (5, 3) ...but how do you find out the radius?
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  2. #2
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    Quote Originally Posted by Joker37 View Post
    The points (8, 4) and (2, 2) are the ends of a diameter of a circle. Find the coordinates of the centre and the radius.


    I know the centre is (5, 3) ...but how do you find out the radius?
    Use the distance formula:
    d = \sqrt{(x_2 - x_1)^2 + (y_2 - y_1)^2}

    Let the (2, 2) be the point (x1, y1) and (8, 4) be (x2, y2):

    \begin{aligned}<br />
d &= \sqrt{(x_2 - x_1)^2 + (y_2 - y_1)^2} \\<br />
d &= \sqrt{(8 - 2)^2 + (4 - 2)^2} \\<br />
d &= \sqrt{6^2 + 2^2} \\<br />
d &= \sqrt{40} \\<br />
 d &= 2\sqrt{10} \\<br />
 \end{aligned}

    Now, this is the diameter, so take half of that to get the radius:
    r = \sqrt{10}


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  3. #3
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    Quote Originally Posted by yeongil View Post
    Use the distance formula:
    d = \sqrt{(x_2 - x_1)^2 + (y_2 - y_1)^2}
    Forgot about the distance formula.
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