Áp dụng tính chất của dãy tỉ số bằng nhau,ta có :
\(\frac{a}{671b+c}=\frac{b}{671c+a}=\frac{c}{671a+b}=\frac{a+b+c}{\left(671b+c\right)+\left(671c+a\right)+\left(671a+b\right)}=\frac{a+b+c}{672.\left(a+b+c\right)}=\frac{1}{672}\)
\(\frac{a}{671b+c}=\frac{1}{672}\Rightarrow672a=671b+c\)
\(\frac{b}{671c+a}=\frac{1}{672}\Rightarrow672b=671c+a\)
\(\frac{c}{671a+b}=\frac{1}{672}\Rightarrow672c=671a+b\)
\(\Rightarrow A=\frac{671b+c}{a}+\frac{671c+a}{b}+\frac{671a+b}{c}\)
\(A=\frac{672a}{a}+\frac{672b}{b}=\frac{672c}{c}=671a+671b+671c=671\left(a+b+c\right)\)