Thursday, November 20, 2025

Geometry Problem 1609: Radius of a Semicircle Tangent to Two Sides of a Triangle

Challenging Geometry Problem 1609. Share your solution by posting it in the comment box provided.
Audience: Mathematics Education - K-12 Schools, Honors Geometry, and College Level.

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Illustration of Geometry Problem 1609

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1 comment:

  1. Very straightforward and obvious solution

    Let O be the centre of the semi-circle
    Let S(ABC) = S

    Then S = S(AOB) + S(COB) = rc/2 + ra/2
    Hence S = r(a+c)/2 and r = 2S/(a+c) .......(1)

    But S = V{s(s-a)(s-b)(s-c) ....... (2) as is well known

    The required result follows easily from (1) and (2)

    Sumith Peiris
    Moratuwa
    Sri Lanka

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