Proving Triangle Angles Equation: cos^2 A + cos^2 B + cos^2 C + 2*cosA*cosB*cosC

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SUMMARY

The equation cos² A + cos² B + cos² C + 2*cosA*cosB*cosC = 1, where A, B, and C are the angles of a triangle, can be proven using the Cosine Rule. The Cosine Rule relates the angles of a triangle to its sides, allowing for substitution of angle values. The discussion emphasizes the need for algebraic manipulation to simplify the equation effectively.

PREREQUISITES
  • Understanding of triangle properties and angle relationships
  • Familiarity with the Cosine Rule
  • Basic algebraic manipulation skills
  • Knowledge of trigonometric identities
NEXT STEPS
  • Study the Cosine Rule in detail to understand its application in triangle geometry
  • Practice algebraic manipulation of trigonometric identities
  • Explore proofs of other trigonometric equations involving triangle angles
  • Learn about the relationship between angles and sides in non-right triangles
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Students studying geometry, particularly those focusing on trigonometry and triangle properties, as well as educators looking for effective teaching methods for proving trigonometric identities.

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Homework Statement


I need to prove the following equation:
cos^2 A + cos^2 B + cos^2 C + 2*cosA*cosB*cosC = 1
where A,B,C are the angles of a triangle

Homework Equations


A+B+C = 180


The Attempt at a Solution


I substituted the sum of A,B,C into the equation, and now I have something like
... + cos^2(180-A-C)+... so on, but I don't know what to do next.

Thanks
 
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Welcome to Physicsforums ER901!

How about using the Cosine rule to express the Cosines as functions of the sides? there's some messy algebra but it should work out fine.
 

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