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Let $a,b,c$ be the sides of a triangle in any random order and let $A,B,C$ be the angles opposite to these sides respectively. Let the inradius be $r$ and circumradius be $R$. In this question, it was proved that

$$ \frac{a}{A} + \frac{b}{B} \ge \frac{c}{C} \tag 1 $$

This is similar in form to the classical triangle inequality $a+b \ge c$ which made me wonder if the there exists a general inequality from which both follows. However, obvious suspects such as

$$ \frac{a}{A^x} + \frac{b}{B^x} \ge \frac{c}{C^x} \tag 2 $$ is false in general for $x \ne 0,1$ and I haven't been able to unify the classical triangle inequality with $(1)$ but I observed related inequality which seems to hold for all triangles from which the classical triangle inequality follows as a special case.

Conjecture 1: If $0 \le x \le 2\sqrt{2}/R$ then $$ \frac{a}{A^{rx}} + \frac{b}{B^{rx}} \ge \frac{c}{C^{rx}} $$

Conjecture 2: Let $m = \min(a,b,c)$ be the smallest side of the triangle. If $0 \le x \le 1/R$ then $$ \frac{a}{A^{mx}} + \frac{b}{B^{mx}} \ge \frac{c}{C^{mx}} $$

In both these conjectures, the classical triangle inequality corresponds to the case $x=0$. Can this be proved or disproved?

  • In your conjecture, what is $r$? Because $x$ only appears adjacent to $r$ in your conjecture, by making $r$ large (or negative) we can make the bounds on $x$ irrelevant. I'm guessing that you used "r" initially, switched to $x$, and mis-typed while getting rid of $r$, but I don't want to presume. – John Hughes Oct 31 '24 at 11:21
  • I totally missed "Let $r$ be the inradius"! My apologies. Thank for helping with my bad reading skills. :) – John Hughes Oct 31 '24 at 11:29
  • The problem I have with both conjectures is that if you multiply all sides by the same factor $\lambda$, leaving the angles invariant, the factor $r$ or $m$ is also multiplied by $\lambda$. This makes the constant bounds on $x$ irrelevant. – Gribouillis Oct 31 '24 at 12:43
  • @Gribouillis I realized that I had missed the factor $R$ in the bounds for $x$. I have added it. Thanks for pointing out. – Nilotpal Sinha Oct 31 '24 at 12:49
  • Is there an intuition behind the $2\sqrt2$ in conjecture 1? – Pranay Oct 31 '24 at 13:20

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