2001 JBMO Problems/Problem 3
Let be an equilateral triangle and on the sides and respectively. If (with ) are the interior angle bisectors of the angles of the triangle , prove that the sum of the areas of the triangles and is at most equal with the area of the triangle . When does the equality hold?
We have Similarly Thus . We have , and , so Obviously , so it is sufficient to prove that . The cosine rule applied to gives . Hence also . Thus we have .
So , which is .
In the above problem, prove that .
- Proposed by
Solution to Bonus Question
Let be the intersection of and , so is an angle bisector of triangle . Extend line DE to meet BC at H. Let us define
We have and Thus .
It follows that is a cyclic quadrilateral. So we have , and
So , implying that triangle is an isoceles triangle. So we have .
Also, since , and , it follows that: trianlge ~ triangle
Similarly it can be shown that: trianlge ~ triangle .
From trianlge ~ triangle we get: , or
From triangle ~ triangle we get: , or
Since , we get
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