MHB Prove one of the angles of triangle is 60°

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Given the relationship between the altitudes and the sides of triangle PQR, the equation 9PA + 4QB + 7RC = 0 indicates that the vectors representing these altitudes form a triangle with sides in the ratio 9a : 4b : 7c. This similarity implies that the angles of triangle PQR correspond to those of the triangle formed by the altitudes. By applying the cosine rule to triangle PQR, it is determined that cos R equals 1/2, leading to the conclusion that angle R is 60 degrees. The discussion highlights the geometric relationships and ratios that confirm this angle measurement.
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Given that $PA,\,QB,\,RC$ are the altitudes of the acute triangle $PQR$ such that $9\vec{PA}+4\vec{QB}+7\vec{RC}=0$.

Show that one of the angles of triangle $PQR$ is $60^{\circ}$.
 
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[sp]Denote the lengths of the sides of the triangle by $|\vec{QR}| = p$, $|\vec{RP}| = q$, $|\vec{PQ}| = r$, and the lengths of the altitudes by $|\vec{PA}| = a$, $|\vec{QB}| = b$, $|\vec{RC}| = c.$ Then the area of the triangle is $\frac12ap = \frac12bq = \frac12cr$, so that the ratios $\frac1a : \frac1b:\frac1c$ and $p:q:r$ are the same.

If $9\vec{PA}+4\vec{QB}+7\vec{RC}=0$, then the vectors $9\vec{PA}$, $4\vec{QB}$ and $7\vec{RC}$ form a triangle whose sides have lengths in the ratio $9a : 4b : 7c$. Also, each side of this triangle is in a direction perpendicular to the corresponding side of the triangle $PQR$ ($\vec{PA}$ is perpendicular to $\vec{BC}$ and so on), so the two triangles have the same angles and are therefore similar. Thus the lengths of their sides are in the same ratio, so that the ratios $9a : 4b : 7c$ and $\frac1a : \frac1b:\frac1c$ are the same. Therefore $9a^2 = 4b^2 = 7c^2$, or equivalently $p^2 = 9\lambda$, $q^2 = 4\lambda$, $r^2 = 7\lambda$ for some constant $\lambda.$

By the cosine rule in the triangle $PQR$, $\cos R = \dfrac{p^2 + q^2 - r^2}{2pq} = \dfrac{9+4-7}{2\sqrt9\sqrt4} = \dfrac6{12} = \dfrac12$ and hence $\angle R = 60^\circ.$[/sp]
 
Thank you Opalg for your neat solution!

Alternate solution that I saw somewhere online:

Let $\alpha=\angle QPR,\,\beta=\angle RQP$ and $\gamma=\angle PRQ$ and $a,\,b,\,c$ be the respective magnitudes of vectors $\vec{PA},\,\vec{QB},\,\vec{RC}$.

Taking the dot product of the vector equation with $\vec{QR}$ and note that $\angle BQR=90^{\circ}-\beta$, we find that $4b\sin \gamma=7c\sin \beta$.

Similarly, $9a\sin \gamma=7c\sin \alpha$ and $9a\sin \beta=4b\sin \alpha$.

Using the conventional notation for the sides of the triangle, we have that $p:q:r=\sin alpha:\sin \beta:\sin \gamma=9a:4b:7c$.

However, we also have that twice the area of triangle $PQR$ is equal to $pa=qb=rc$, so that $p:q:r=\dfrac{1}{a}:\dfrac{1}{b}:\dfrac{1}{c}$. Therefore $9a^2=4b^2=7c^2=k$ for some constant $k$.

$\begin{align*}\therefore \cos PRQ&=\dfrac{p^2+q^2-r^2}{2pq}\\&=\dfrac{81a^2+16b^2-49c^2}{72ab}\\&=\dfrac{9k+4k-7k}{12k}\\&=\dfrac{1}{2}\end{align*}$

from which it follows that $\angle R=60^{\circ}$.
 
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