Triangle Inequality Proof: $AD+DC \le AB+BC$ for Point D in Triangle ABC

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In summary, the Triangle Inequality Theorem states that the sum of any two sides of a triangle must be greater than the length of the third side. This theorem can be applied to any point on a line segment in a triangle, and it helps to determine if a given set of side lengths can form a valid triangle. It is also related to the Pythagorean Theorem, which is a more specific case for right triangles. The Triangle Inequality Theorem applies to all types of triangles, making it a fundamental concept in geometry.
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Jameson
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Thank you to Ackbach for this problem and to those of you who participated in last week's POTW!

Given a triangle $ABC$ and a point $D$ inside $ABC$, prove that $\overline{AD}+\overline{DC}\le \overline{AB}+\overline{BC}$.
 
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Congratulations to the following members for their correct solutions:

1) caffeinemachine

Solution:

[sp]Extend line $AD$ such that it intersects with line $BC$ at point $E$. Use the triangle inequality twice:

$$\overline{AD}+\overline{DC}\le \overline{AD}+\overline{DE}+\overline{EC}$$

$$=\overline{AE}+\overline{EC}\le \overline{AB}+\overline{BE}+\overline{EC}$$

$$=\overline{AB}+\overline{BC}.$$

QED[/sp]
 

Related to Triangle Inequality Proof: $AD+DC \le AB+BC$ for Point D in Triangle ABC

What is the Triangle Inequality Theorem?

The Triangle Inequality Theorem states that the sum of any two sides of a triangle must be greater than the length of the third side.

How can the Triangle Inequality Theorem be applied to Point D in Triangle ABC?

Point D can be any point on the line segment AC, and the Triangle Inequality Theorem can be used to prove that the sum of the lengths of the two smaller sides, AD and DC, is always less than or equal to the sum of the lengths of the two larger sides, AB and BC.

What is the importance of the Triangle Inequality Theorem?

The Triangle Inequality Theorem is important because it helps to determine whether a given set of side lengths can form a valid triangle. It is also a fundamental concept in geometry and is used in many other proofs and theorems.

How is the Triangle Inequality Theorem related to the Pythagorean Theorem?

The Triangle Inequality Theorem is a generalization of the Pythagorean Theorem, which only applies to right triangles. The Pythagorean Theorem states that the square of the length of the hypotenuse (the longest side) is equal to the sum of the squares of the other two sides. This can also be written as c² = a² + b², where c is the length of the hypotenuse and a and b are the lengths of the other two sides.

Can the Triangle Inequality Theorem be applied to any type of triangle?

Yes, the Triangle Inequality Theorem can be applied to any type of triangle, including equilateral, isosceles, and scalene triangles. It is a general rule that applies to all triangles, regardless of their size or shape.

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