Maths Olympiad Prep

Library / /294 of 520

Algebra Difficulty 6.8 National olympiad Prove it

In [1], the RRR \cdot R \cdot Janic inequality is given by
rahb+hc+rbhc+ha+rcha+hb32.\frac{r_{a}}{h_{b}+h_{c}}+\frac{r_{b}}{h_{c}+h_{a}}+\frac{r_{c}}{h_{a}+h_{b}} \geqslant \frac{3}{2} .

In [2], the inequality is strengthened to
rahb+hcrbhc+harcha+hb18\frac{r_{a}}{h_{b}+h_{c}} \frac{r_{b}}{h_{c}+h_{a}} \frac{r_{c}}{h_{a}+h_{b}} \geqslant \frac{1}{8}

Here, ha,hb,hc,ra,rb,rch_{a}, h_{b}, h_{c}, r_{a}, r_{b}, r_{c} represent the altitudes and the radii of the excircles opposite to the sides a,b,ca, b, c of ABC\triangle ABC, respectively.

Solution

2bcs(sa)2bc=s(sa)\leqslant \frac{2 \sqrt{b c s(s-a)}}{2 \sqrt{b c}}=\sqrt{s(s-a)}

It can be known that rh+rc2wcr_{h}+r_{c} \geqslant 2 w_{c},
Similarly, rc+ru2wb,ru+rh2wcr_{c}+r_{u} \geqslant 2 w_{b}, r_{u}+r_{h} \geqslant 2 w_{c}.
Thus, inequalities (3) and (4) can be strengthened to
(warh+rc)λ+(wbrc+ra)+(wcru+rh)λ32λ(λR+)warh+rcwbrc+rawcra+rh18\begin{array}{l} \left(\frac{w_{a}}{r_{h}+r_{c}}\right)^{\lambda}+\left(\frac{w_{b}}{r_{c}+r_{a}}\right)^{\prime}+\left(\frac{w_{c}}{r_{u}+r_{h}}\right)^{\lambda} \\ \leqslant \frac{3}{2^{\lambda}}\left(\lambda \in R^{+}\right) \\ \frac{w_{a}}{r_{h}+r_{c}} \frac{w_{b}}{r_{c}+r_{a}} \frac{w_{c}}{r_{a}+r_{h}} \leqslant \frac{1}{8} \end{array}

This paper provides similar and strengthened versions of (1) and (2).
rb+rc=s(sc)(sa)sb+s(sa)(sb)sc2s(sa),(s=a+b+c2)ha=2s(sa)(sb)(sc)a=s(sa)1a2(sb)(sc)s(sa)[(sb)+(sc)]a=s(sa),rh+rc2ha\begin{array}{l} \because r_{b}+r_{c} \\ =\sqrt{\frac{s(s-c)(s-a)}{s-b}}+\sqrt{\frac{s(s-a)(s-b)}{s-c}} \\ \geqslant 2 \sqrt{s(s-a)}, \quad\left(s=\frac{a+b+c}{2}\right) \\ h_{a}=\frac{2 \sqrt{s(s-a)(s-b)(s-c)}}{a} \\ =\sqrt{s(s-a)} \cdot \frac{1}{a} \cdot 2 \sqrt{(s-b)(s-c)} \\ \leqslant \frac{\sqrt{s(s-a)}[(s-b)+(s-c)]}{a} \\ =\sqrt{s(s-a)}, \\ \therefore r_{h}+r_{c} \geqslant 2 h_{a} \text {. } \end{array}

Similarly, we have
rc+ru2hb.ra+rh2hc.r_{c}+r_{u} \geqslant 2 h_{b} . \quad r_{a}+r_{h} \geqslant 2 h_{c} .

From this, it is easy to obtain the similar inequalities of (1) and (2):
harb+rc+hbrc+ra+hcra+rb32harb+rchbrc+rahcra+rh18\begin{array}{l} \frac{h_{a}}{r_{b}+r_{c}}+\frac{h_{b}}{r_{c}+r_{a}}+\frac{h_{c}}{r_{a}+r_{b}} \leqslant \frac{3}{2} \\ \frac{h_{a}}{r_{b}+r_{c}} \frac{h_{b}}{r_{c}+r_{a}} \frac{h_{c}}{r_{a}+r_{h}} \leqslant \frac{1}{8} \end{array}

Let the angle bisectors of ABC\triangle A B C at angles AA, BB, and CC be waw_{a}, wbw_{b}, and wcw_{c}, respectively, where
wa=2bcs(sa)b+cw_{a}=\frac{2 \sqrt{b c s(s-a)}}{b+c}
- 16 -

Want a route through all this instead of an archive? The track puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.

Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.