MHB Equlateral Triangle: Prove $ab \cos \gamma = ac \cos \beta = bc \cos \alpha$

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To prove that a triangle is equilateral if and only if \( ab \cos \gamma = ac \cos \beta = bc \cos \alpha \), two methods are suggested. The first method involves using the cosine values derived from the triangle's angles and applying them to the sides. The second method also utilizes cosine values but approaches the proof from a different angle, reinforcing the relationship among the sides and angles. The discussion emphasizes the equivalence of these conditions in establishing the equilateral nature of the triangle. Overall, the proofs demonstrate the fundamental relationship between the sides and angles in an equilateral triangle.
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Given a triangle with angles $\alpha, \beta, \gamma$ opposite respectively to the sides $a,b,c$. Show in at least two different ways, that the triangle is equilateral if and only if $ab \cos \gamma = ac \cos \beta = bc \cos \alpha$.
 
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lfdahl said:
Given a triangle with angles $\alpha, \beta, \gamma$ opposite respectively to the sides $a,b,c$. Show in at least two different ways, that the triangle is equilateral if and only if $ab \cos \gamma = ac \cos \beta = bc \cos \alpha$.

Method 1)

$ab \cos \gamma = ac \cos \beta = bc \cos \alpha$
hence $\frac{1}{c}\cos \gamma =\frac{1}{b} \cos \beta = \frac{1}{a} \cos \alpha$
we have from sin rule $\frac{1}{c}\sin \gamma =\frac{1}{b} \sin \beta = \frac{1}{a} \sin \alpha$
square and add to get $\frac{1}{c^2} =\frac{1}{b^2} = \frac{1}{a^2} $ or $a=b=c$ hence equilateral

for the other part if $a=b=c$ then
$ \alpha = \beta = \gamma$
hence $ \cos \alpha = \cos \beta = \cos \gamma$ and by multiplying we get the result

method 2

using the values of cos we get
$ab \cos \gamma = ac \cos \beta = bc \cos \alpha$
$<==> a^2+b^2-c^2 = a^2+c^2-b^2= b^2 + c^2 - a^2$
from $a^2+b^2-c^2 = a^2+c^2-b^2$
$<==> b^2-c^2<==>b=c$ simlilarly a =c $<==>$ it is equilateral
 
kaliprasad said:
Method 1)

$ab \cos \gamma = ac \cos \beta = bc \cos \alpha$
hence $\frac{1}{c}\cos \gamma =\frac{1}{b} \cos \beta = \frac{1}{a} \cos \alpha$
we have from sin rule $\frac{1}{c}\sin \gamma =\frac{1}{b} \sin \beta = \frac{1}{a} \sin \alpha$
square and add to get $\frac{1}{c^2} =\frac{1}{b^2} = \frac{1}{a^2} $ or $a=b=c$ hence equilateral

for the other part if $a=b=c$ then
$ \alpha = \beta = \gamma$
hence $ \cos \alpha = \cos \beta = \cos \gamma$ and by multiplying we get the result

method 2

using the values of cos we get
$ab \cos \gamma = ac \cos \beta = bc \cos \alpha$
$<==> a^2+b^2-c^2 = a^2+c^2-b^2= b^2 + c^2 - a^2$
from $a^2+b^2-c^2 = a^2+c^2-b^2$
$<==> b^2-c^2<==>b=c$ simlilarly a =c $<==>$ it is equilateral
Well done, kaliprasad! Thankyou for your participation!(Cool)
 
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