Who is as Clever as a Canadian?

The Canadian Open Math Challenge recently asked Canadian high school students to find the ratio of the inradii of two triangles ADP and CDP, given that ABCD is a parallelogram, that the circle inscribed in ABC is tangent to AC at P, that AD+DC=3(AC), and that AD=DP.

Can you match our Canadian students and find that ratio?


Here is a diagram:


Not to scale of course!

1 Answer

  • Duke
    Lv 7
    8 years ago
    Favorite Answer

    I got 4/3. These Canadians are very clever indeed!

    If we take |AD| = |DP| = 1, |DC| = a > 1 (i.e. up to similarity) and the property of each touching point of the incircle of ∆ABC being at distances to the vertices, equal to the semi-perimeter minus the opposing side, we can easily find

    |AP| = (2a - 1)/3 and |PC| = (2 - a)/3 using |AC| = (1 + a)/3 (by the way 1< a < 5/4).

    Now the calculation shows ∆ADP and ∆CDP have equal semi-perimeters, so the ratio in question is equal to the ratio of their areas, the latter in turn equal to the ratio

    |AP| : |PC|. Dropping the perpendicular from D to AC and expressing the common altitude of the both aforementioned triangles from 2 right triangles with common leg we arrive to equation

    10a² - a - 11 = 0, hence a = 1.1 and the rest is clear.

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