Áp dụng t/c dtsbn:
\(\dfrac{2021a+b+c}{a}=\dfrac{a+2021b+c}{b}=\dfrac{a+b+2021c}{c}=\dfrac{2023\left(a+b+c\right)}{a+b+c}=2023\)
\(\Rightarrow\left\{{}\begin{matrix}2023a=2021a+b+c\\2023b=a+2021b+c\\2023c=a+b+2021c\end{matrix}\right.\)\(\Rightarrow\left\{{}\begin{matrix}2a=b+c\\2b=a+c\\2c=a+b\end{matrix}\right.\)
\(S=\dfrac{a+b}{c}+\dfrac{b+c}{a}+\dfrac{c+a}{b}=\dfrac{2c}{c}+\dfrac{2a}{a}+\dfrac{2b}{b}=2+2+2=6\)