Nocturne No. 2

Geometry Level 4

The diagram shows a large triangle with a green side of length 10, a blue side of length 17, and an inradius of 3.5.

At the point of tangency of the large circle and the large triangle, two other lines are drawn, parallel to the triangle.

Find the sum of the radii of the two smaller incircles.


The answer is 3.5.

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1 solution

Lolly Lau
Jun 21, 2016

Relevant wiki: Solving Triangles - Problem Solving - Medium

Nocturne No.2 Commentary

Notice how the blue, red and large triangles are similar.

Therefore, the ratio between the inradius and the length of the green side of each triangle is the same.

Also, the sum of the lengths of the green sides of the small triangles

is equal to the length of the green side of the large triangle.

Therefore, the sum of the inradii of the small triangles is equal to the inradius of the large triangle.

Oh, you could have simply edited the previous question. I have deleted it now. Thanks!

Calvin Lin Staff - 4 years, 11 months ago

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Oh :) Thanks!

Lolly Lau - 4 years, 11 months ago

Solved absolutely on the same principle.

Niranjan Khanderia - 4 years, 8 months ago

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Good for you :)

Lolly Lau - 4 years, 8 months ago

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