In triangle $ABC$, the medians and bisectors corresponding to sides $BC$, $CA$, $AB$ are $m_a$, $m_b$, $m_c$ and $w_a$, $w_b$, $w_c$ respectively. $P=w_a \cap m_b$, $Q=w_b \cap m_c$, $R=w_c \cap m_a$. Denote the areas of triangle $ABC$ and $PQR$ by $F_1$ and $F_2$ respectively. Find the least positive constant $m$ such that $\frac{F_1}{F_2}<m$ holds for any $\triangle{ABC}$.
Problem
Source: China TST 2003
Tags: geometry, ratio, inequalities, analytic geometry, linear algebra, matrix, geometry unsolved
19.05.2009 10:00
I would guess it's "...find the least $ m$ such that $ \frac{F_2}{F_1} < m$..." because there is no such $ m$ as it is stated. Areal coordinates: any point on $ w_a$ has coordinates $ \left(\tfrac{x}{x+b+c}, \tfrac{b}{x+b+c}, \tfrac{c}{x+b+c}\right)$, where $ x$ is positive; any point on $ m_b$ has coordinates $ \left(\tfrac{1}{2+y}, \tfrac{y}{2+y}, \tfrac{1}{2+y}\right)$, where $ y$ is positive. Using this we can find that the intersection $ w_a \cap m_b$ has areal coordinates $ \left(\tfrac{c}{b+2c}, \tfrac{b}{b+2c}, \tfrac{c}{b+2c}\right)$. Similarly, $ w_b \cap m_c$ has areal coordinates $ \left(\tfrac{a}{c+2a}, \tfrac{a}{c+2a}, \tfrac{c}{c+2a}\right)$ and $ w_c \cap m_a$ has areal coordinates $ \left(\tfrac{a}{a+2b}, \tfrac{b}{a+2b}, \tfrac{b}{a+2b}\right)$. Hence \[ \frac{F_2}{F_1} = \left|\begin{matrix} \tfrac{c}{b+2c} & \tfrac{b}{b+2c} & \tfrac{c}{b+2c} \\ \tfrac{a}{c+2a} & \tfrac{a}{c+2a} & \tfrac{c}{c+2a} \\ \tfrac{a}{a+2b} & \tfrac{b}{a+2b} & \tfrac{b}{a+2b} \end{matrix}\right| = \frac{ab^2+bc^2+ca^2-3abc}{(a+2b)(b+2c)(c+2a)}\] I would guess that the maximum of the above expression occurs when $ a,b=x, c\to 2x$, which gives $ m = \tfrac{1}{60}$? Otherwise, it is just tedious taking of derivatives.
09.04.2016 06:48
Actually, $ a,b=x, c\to 0$ gives $m=\frac{1}{6}$, which turns out to be the maximum.