Question 101
Find all positive real solutions $(a, b, c, d)$ of equation $$
\frac{a^{2}-b d}{b+2 c+d}+\frac{b^{2}-c a}{c+2 d+a}+\frac{c^{2}-d b}{d+2 a+b}+\frac{d^{2}-a c}{a+2 b+c}=0
$$
\frac{a^{2}-b d}{b+2 c+d}+\frac{b^{2}-c a}{c+2 d+a}+\frac{c^{2}-d b}{d+2 a+b}+\frac{d^{2}-a c}{a+2 b+c}=0
$$