Triangle inequality proof in Spivak's calculus

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SUMMARY

The forum discussion centers on the proof of the triangle inequality as presented in Spivak's "Calculus." A participant expresses confusion regarding the transition from the squared terms to the inequality |a+b| ≤ |a| + |b|. They question why Spivak does not consider the case of equality in the transition from x² < y² to x < y, suggesting that the proof lacks completeness. The discussion highlights the importance of understanding the conditions under which inequalities hold in the context of natural numbers.

PREREQUISITES
  • Understanding of basic calculus concepts, particularly inequalities.
  • Familiarity with the triangle inequality theorem.
  • Knowledge of Spivak's "Calculus" and its mathematical proofs.
  • Basic algebraic manipulation skills, including factorization and square roots.
NEXT STEPS
  • Study the triangle inequality in detail, focusing on its proof and applications.
  • Review the properties of inequalities in the context of natural numbers.
  • Examine Spivak's "Calculus" for additional examples of proofs involving inequalities.
  • Learn about the implications of equality in mathematical inequalities.
USEFUL FOR

Students of calculus, mathematicians interested in proofs, and anyone seeking a deeper understanding of inequalities in mathematical contexts.

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So hi, there's one little thing which I'm not understanding in the proof. After the inequality Spivak considers the two expressions to be equal. Why?!?

I just don't see why we can't continue with the inequality and when we have factorized the identity to (|a|+|b|)^2 we can just replace (a+b)^2 with (|a+b|)^2 and take the square root of both sides to finally have :

|a+b| <= |a|+|b|

Thank you for explaining !
 
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No need to answer, it's understood !
 
You could also start from ##-|a| \leq a \leq |a|##.
 
Well, I have another question. When Spivak justifies the passage from the squares to |a+b| <= |a|+|b| he says the following : x^2<y^2 supposes that x<y for x,y in N. Now, the only thing bugging me is the following : Why didn't he do the following x^2<=y^2 supposes that x<=y for x,y in N ? Because what he says only justifies the inequality and not the equality ! Like a part is missing ! Am I right ? Thank you!
 

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