Results 1 to 2 of 2

Math Help - Radius of circle inside a circle

  1. #1
    Junior Member
    Joined
    Oct 2007
    Posts
    68

    Radius of circle inside a circle

    I am stumped with this Grade 11 circle challenge.

    I'm supposed to find 'r' but no measurements are given. Can I get clues?



    thanks!!
    Follow Math Help Forum on Facebook and Google+

  2. #2
    MHF Contributor red_dog's Avatar
    Joined
    Jun 2007
    From
    Medgidia, Romania
    Posts
    1,251
    Thanks
    3
    O=the center of the big circle. O_1=the center of the small circle.

    Let OA, \ OB the radius tangent to the small circle. In the figure it seems that the radius are perpendicular.

    Let OC\perp OA.

    In the triangle OCO_1 apply Pitagora:

    OO_1^2=OC^2+O_1C^2\Rightarrow (R-r)^2=r^2+r^2\Rightarrow r^2-2Rr-R^2=0\Rightarrow r=R(\sqrt{2}-1)
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. Area inside cardioid and also inside circle??
    Posted in the Calculus Forum
    Replies: 3
    Last Post: April 19th 2010, 05:57 PM
  2. Replies: 7
    Last Post: March 15th 2010, 04:10 PM
  3. Replies: 2
    Last Post: February 6th 2010, 08:31 AM
  4. Circle inside Circle (ask)
    Posted in the Trigonometry Forum
    Replies: 1
    Last Post: January 19th 2010, 10:38 AM
  5. Radius of circle inside right triangle
    Posted in the Geometry Forum
    Replies: 3
    Last Post: November 7th 2009, 11:53 PM

Search Tags


/mathhelpforum @mathhelpforum