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Haboric

As Professor mentioned in class, this is called triangle inequality. "Pentagon inequality" reminds us of thinking while reading notes.

Animagus

Say if we have a, b and c satisfy a triangle.

a + b > c

|a - b| < c

geminish

What's the meaning of |a-b|<c? I think a transform of first inequality should be |c-a|<b?

PPCC

@geminishActually we have some symmetricity of the triangle inequality. That is to say, we have |c-a|<b, |a-b|<c and |b-c|<a. Basically we should think that all edges have equal role and none of them are special and singular. For @Animagus, I guess he/she just wants to point out the relationship between (a,b) and c. The second inequality does not directly come from the first one.