Let be a trapezoid with parallel to , and equal edges . Let be the center of the circle tangent to lines and , where and are on opposite sides of . Let be the center of the circle tangent to lines and , where and are on opposite sides of . Prove that .
Solutions — 2
Solution 1
Let and let . Since is an isosceles trapezoid, is an isosceles triangle. Therefore , which implies that since lies on the external bisector of . Since lies on the bisector of , it follows that and hence that is isosceles with . Similarly is isosceles with . So, in the triangles and we have and . Since and both lie on the internal bisector of , it follows that triangles and are congruent. Therefore .
A competition of the Canadian Mathematical Society and supported by the Actuarial Profession.
!
Expertise. Insight. Solutions.
Solution 2
1. Identify the properties of the trapezoid:
- Given is a trapezoid with and .
- The edges are equal, indicating that is an isosceles trapezoid.
2. Intersection of diagonals:
- Let . In an isosceles trapezoid, the diagonals intersect at a point such that and .
3. Angles at the intersection point:
- Since , the angles and .
4. Centers of the circles:
- Let be the center of the circle tangent to lines , , and .
- Let be the center of the circle tangent to lines , , and .
5. Angles involving the centers:
- Since is the center of the circle tangent to , , and , the angle because is tangent to the circle with center .
- Similarly, implies .
6. Equal distances from the intersection point:
- From the above, we have because and is the midpoint of and in terms of the tangency points.
7. **Similar argument for :**
- By a similar argument, because and is the midpoint of and in terms of the tangency points.
8. Congruent triangles:
- We also have because both are formed by the intersection of the diagonals and the tangency points.
- Combining , , and , we get by the Angle-Side-Angle (ASA) criterion.
9. Conclusion:
- Since , it follows that .
The final answer is