MHB Proving Triangle Angles Relation: $\frac{1}{\sin^2 \lambda}$

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In triangle ABC, a point P exists such that angles PAB, PBC, and PCA are equal to λ. The relationship to prove is that 1/sin²λ equals the sum of the reciprocals of the squares of the sines of the triangle's angles α, β, and γ. The discussion includes a detailed proof of this relationship using trigonometric identities and properties of triangles. Participants engage in exploring various approaches to validate the equation. The proof highlights the interconnectedness of the angles and their sine values within the triangle's geometry.
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Inside a triangle $ABC$, there is a point P satisfies $\angle PAB=\angle PBC=\angle PCA=\lambda$. If the angles of the triangle are denoted by $\alpha$, $\beta$ and $\gamma$, prove that

$\dfrac{1}{\sin^2 \lambda}=\dfrac{1}{\sin^2 \alpha}+\dfrac{1}{\sin^2 \beta}+\dfrac{1}{\sin^2 \gamma}$
 
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anemone said:
Inside a triangle $ABC$, there is a point P satisfies $\angle PAB=\angle PBC=\angle PCA=\lambda$. If the angles of the triangle are denoted by $\alpha$, $\beta$ and $\gamma$, prove that

$\dfrac{1}{\sin^2 \lambda}=\dfrac{1}{\sin^2 \alpha}+\dfrac{1}{\sin^2 \beta}+\dfrac{1}{\sin^2 \gamma}$
using the sine law:
View attachment 1935
 

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Last edited:
Thanks for participating, Albert!

Solution provided by other:
View attachment 1936
First we let AP meet BC at X.

Since $\angle XBP=\angle BAX=\lambda$ and $\angle BXP=\angle AXB=\text{common angle}$, we can say that triangle XPB and XBA are similar. Then we have $\dfrac{XB}{XP}=\dfrac{XA}{XB}$

Applying the sine law to the triangle XBA and considering the fact that $\dfrac{XB}{XP}=\dfrac{XA}{XB}$ give us

$\dfrac{\sin \lambda}{\sin \beta}=\dfrac{XB}{XA}$

$\dfrac{\sin^2 \lambda}{\sin^2 \beta}=\dfrac{XB^2}{XA^2}=\dfrac{XAXP}{XA^2}= \dfrac{XP}{XA}$

Now, the ratio of the area of the triangles XPB to XBA and triangles XCP to XCA are

$\dfrac{\text{area of the triangle XPB}}{\text{area of the triangle XBA}}=\dfrac{XP}{XA}$ and

$\dfrac{\text{area of the triangle XCP}}{\text{area of the triangle XCA}}=\dfrac{XP}{XA}$

This tells us $\dfrac{\text{area of the triangle XPB}}{\text{area of the triangle XBA}}=\dfrac{\text{area of the triangle XCP}}{\text{area of the triangle XCA}}=\dfrac{\text{area of the triangle BPC}}{\text{area of the triangle ABC}}$

$\therefore \dfrac{\sin^2 \lambda}{\sin^2 \beta}=\dfrac{\text{area of the triangle BPC}}{\text{area of the triangle ABC}}$

By the similar arguments, we have

$\therefore \dfrac{\sin^2 \lambda}{\sin^2 \alpha}=\dfrac{\text{area of the triangle APB}}{\text{area of the triangle ABC}}$

$\therefore \dfrac{\sin^2 \lambda}{\sin^2 \gamma}=\dfrac{\text{area of the triangle CPA}}{\text{area of the triangle ABC}}$

Hence,

$\small\dfrac{\sin^2 \lambda}{\sin^2 \alpha}+\dfrac{\sin^2 \lambda}{\sin^2 \beta}+\dfrac{\sin^2 \lambda}{\sin^2 \gamma}=\dfrac{\text{area of the triangle APB}}{\text{area of the triangle ABC}}+\dfrac{\text{area of the triangle BPC}}{\text{area of the triangle ABC}}+\dfrac{\text{area of the triangle CPA}}{\text{area of the triangle ABC}}=1$

$\dfrac{1}{\sin^2 \lambda}=\dfrac{1}{\sin^2 \alpha}+\dfrac{1}{\sin^2 \beta}+\dfrac{1}{\sin^2 \gamma}$ (Q.E.D.)
 

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