AO+BO+CO≥6r where r is the radius of the inscribed circle

The goal is to prove that the sum of the distances from the center of the inscribed circle to each vertex of the triangle is greater than or equal to six times the radius of the circle.
  • #1
solakis1
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From the entrance examinations to Ghana University ,from high school, i got the following problem:

If O is the center of the inscribed circle in an ABC trigon,then prove that: \(\displaystyle AO+BO+CO\geq 6r\) where r is the radius of the inscribed circle.
 
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  • #2
I first thought that your "ABC trigon" is what I would call a "triangle" but that would make the problem impossible. If ABC is a triangle inscribed in a circle, of radius r, then OA, OB, and OC are equal to r so that OA+ OB+ OC= 3r which is less than 6r.
 
  • #3
HallsofIvy said:
I first thought that your "ABC trigon" is what I would call a "triangle" but that would make the problem impossible. If ABC is a triangle inscribed in a circle, of radius r, then OA, OB, and OC are equal to r so that OA+ OB+ OC= 3r which is less than 6r.

The question is about the circle inscribed in a triangle, not about a triangle inscribed in a circle.
 

1. What is the meaning of the equation AO+BO+CO≥6r where r is the radius of the inscribed circle?

The equation AO+BO+CO≥6r is known as the triangle inequality theorem, which states that the sum of any two sides of a triangle must be greater than or equal to the length of the third side. In this case, the equation is specifically referring to the relationship between the lengths of the three sides of a triangle and the radius of its inscribed circle.

2. How is the equation AO+BO+CO≥6r used in geometry?

The equation is used to determine whether a triangle is possible with given side lengths. If the sum of any two sides is less than the length of the third side, then the triangle cannot exist. Additionally, the equation is also used to calculate the radius of the inscribed circle in a triangle.

3. What is an inscribed circle in a triangle?

An inscribed circle is a circle that is tangent to all three sides of a triangle. It is the largest possible circle that can be drawn within a triangle, and its center is called the incenter. The radius of the inscribed circle is denoted by r in the equation AO+BO+CO≥6r.

4. How is the radius of the inscribed circle calculated?

The radius of the inscribed circle can be calculated using the equation r = A/2s, where A is the area of the triangle and s is the semiperimeter (half the perimeter) of the triangle. Alternatively, it can also be calculated using the formula r = Δ/Δs, where Δ is the area of the triangle and Δs is the sum of the lengths of the sides of the triangle.

5. Can the equation AO+BO+CO≥6r be used for all types of triangles?

Yes, the triangle inequality theorem and the equation AO+BO+CO≥6r can be applied to all types of triangles, including equilateral, isosceles, and scalene triangles. However, the results may vary depending on the type of triangle and the given side lengths.

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