Basic Trigonometry: Explaining the Rules

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    Trigonometry
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Discussion Overview

The discussion revolves around the definitions and proofs related to basic trigonometric functions, specifically sine, cosine, and tangent, within the context of planar geometry. Participants explore the reasoning behind these definitions and seek to understand their foundational basis.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants assert that the definitions of sine, cosine, and tangent are simply established definitions in planar geometry.
  • Others inquire about the nature of "explanations" behind these definitions, suggesting a desire for deeper understanding.
  • One participant proposes that sine can be defined using the unit circle, implying a geometric proof based on similar triangles.
  • Another participant emphasizes the need to prove that the ratio of opposite side to hypotenuse remains constant for triangles with the same angle.

Areas of Agreement / Disagreement

Participants express differing views on whether the definitions of trigonometric functions require proof or are simply accepted as definitions. The discussion remains unresolved regarding the necessity of proving these relationships.

Contextual Notes

There is an implicit assumption that the discussion is grounded in planar geometry, but the definitions and proofs may vary based on different mathematical frameworks or contexts.

iknownth
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We all know that
sin theta = opposite side / hypotenuse
cos theta = adjacent side / hypotenuse
tan theta = opposite side / adjacent side.
But why? Are there some explanations behind or are they just defined by scientists?
 
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iknownth said:
But why? Are there some explanations behind or are they just defined by scientists?

What do you mean by "explanations"?

To answer the question, they are the definitions used in planar geometry. There are other (equivalent) ways of defining them, but you'll get the same properties nonetheless.
 
How can one prove that sin theta = opposite side / hypotenuse ?
 
iknownth said:
How can one prove that sin theta = opposite side / hypotenuse ?

I'm assuming planar geometry here. There are two answers:
  1. It is the definition of sine. There is nothing to prove.
  2. The definition is using the unit circle. It which case the proof is immediate from similar triangles.

I guess it's worth asking: what is your definition of sine?
 
pwsnafu said:
It is the definition of sine. There is nothing to prove.

In this case, there is still something to prove. You want to prove also that if two triangles have the same angle, then the quantity opposite side/hypothenuse is the same.
 
micromass said:
In this case, there is still something to prove. You want to prove also that if two triangles have the same angle, then the quantity opposite side/hypothenuse is the same.

Arrgh yes of course.

Where's the brainfart smiley when you need one?
 

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