Understanding the Laws of Sine and Cosine

  • Context: High School 
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    Laws Sine/cosine
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SUMMARY

The discussion centers on the Law of Sine and the Law of Cosines, which are fundamental concepts in trigonometry. The Law of Sine states that the ratio of the length of a side of a triangle to the sine of its opposite angle is constant across all three sides. The Law of Cosines provides a formula to calculate the length of a side in a triangle when the lengths of the other two sides and the included angle are known. Participants express a desire for proofs and examples to better understand these laws, particularly in relation to triangle area calculations and right-angled triangles.

PREREQUISITES
  • Understanding of basic trigonometric functions (sine, cosine)
  • Familiarity with triangle properties and terminology
  • Knowledge of the Pythagorean theorem
  • Ability to manipulate algebraic expressions
NEXT STEPS
  • Study the derivation of the Law of Sine using triangle area formulas
  • Explore the proof of the Law of Cosines using right triangles
  • Practice solving problems involving both laws in various triangle configurations
  • Investigate applications of these laws in real-world scenarios, such as navigation and architecture
USEFUL FOR

Students, educators, and anyone seeking to deepen their understanding of trigonometry, particularly in relation to triangle properties and proofs of fundamental laws.

manbush
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Law of Sine = Sin A/a = Sin B /b = Sin c/c

Law of Cosines
a^2=b^2 + c^2 - 2bc Cos A

I used to know how to "prove" both of these laws but I haven't touched math for years.

Can someone "prove" them or "explain" what these laws exactly mean?
 
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As for the law of sines:
Use the sine law for the AREA of triangles in a clever way!

As for the cosine law:
Make a triangle with acute angles, draw its height h, so that you have two right-angled triangles.
Fiddle around with the Pythogorean theorem until you stumble upon the law of cosine
 
Sorry, I don't understand either. I been away from math too long. I know I could just use these formulas but I want to see the "proof".

Is it possible you could show me an example of proving them?
 
My friend taught me this...dont' quite remember, i would appreciate any help :D
 

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