Area of Triangle with Cross Product: Equation Variations

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Discussion Overview

The discussion revolves around the formula for the area of a triangle using the cross product of two vectors, specifically exploring why the area remains consistent regardless of which two adjacent sides are selected. The scope includes mathematical reasoning and conceptual clarification.

Discussion Character

  • Mathematical reasoning, Conceptual clarification

Main Points Raised

  • One participant seeks to understand the mathematical proof that the area of a triangle, calculated as 1/2 the magnitude of the cross product of two adjacent sides, is invariant to the choice of those sides.
  • Another participant suggests labeling the triangle's vertices to clarify the sides involved in the calculation.
  • A later reply explains that the area derived from the cross product is based on the sine of the angle between the two vectors, noting that the angle will either be θ or π - θ, leading to the same sine value and thus the same area.

Areas of Agreement / Disagreement

Participants have not reached a consensus on a formal proof, and the discussion includes varying levels of understanding and approaches to the problem.

Contextual Notes

The discussion does not resolve the mathematical proof sought by the initial participant, and assumptions regarding the vectors and angles involved remain unexamined.

Neen87
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Hello!

I'm trying to understand how this formula: 1/2 magnitude of v × w (area of triangle)
yields the same value no matter which 2 adjacent sides are chosen.

How would you prove mathematically that this is the case?
 
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Welcome to PF!

Hi Neen87! Welcome to PF! :smile:

Hint: call the vertices a b and c, so the sides are a - b etc. :wink:
 
Neen87 said:
Hello!

I'm trying to understand how this formula: 1/2 magnitude of v × w (area of triangle)
yields the same value no matter which 2 adjacent sides are chosen.

How would you prove mathematically that this is the case?

Because if you draw the parallelogram with sides v and w, the cross product magnitude gives:

|v \times w| = |v||w|\sin\theta

where \theta is the angle between the two vectors you have chosen for sides. Now, whichever two sides you choose and whichever direction they point, the angle between them will be either \theta or \pi - \theta. Either way you get the same value for its sine.
 
Thanks so much! :-)
 

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