$C1$
$\frac{a}{5}$ =$\frac{b}{2}=>5b=2a<=>b=\frac{2}{5}a$
$\frac{b}{-3} =\frac{c}{4}=>-3c=4b<=>c=\frac{-4}{3}b=\frac{-4}{3}b.\frac{2}{5}a=\frac{-8}{15}a$
$a+b-c=2871<=>a+\frac{2}{5}a-\frac{-8}{15}a=2871<=>\frac{29}{15}a=2871$
$=>a=1485 $
=>$\left \{ {b=594} \atop {c=-792} \right.$
$C2:$
$\frac{a}{5} =\frac{b}{2}$
<=>$\frac{3a}{15} =\frac{3b}{6}=\frac{3a+3b}{15+6} =\frac{3(a+b)}{21}=\frac{a+b}{7}(1)$
$\frac{b}{-3} =\frac{c}{4}$
<=>$\frac{2b}{6} =\frac{-2c}{8}=\frac{2b-2c}{6+8} =\frac{2(b-c)}{14}=\frac{b-c}{7}(2)$
Từ $(1)$ và $(2)$
=>$\frac{3b}{6}+\frac{2b}{6}=\frac{a+b}{7}(1)+\frac{b-c}{7}$
$<=>\frac{5b}{6}=\frac{a+b-c+b}{7}$
$<=>\frac{5b}{6}=\frac{a+b-c}{7}+\frac{b}{7}$
$<=>\frac{5b}{6}-\frac{b}{7}=\frac{a+b-c}{7}$
$<=>\frac{29b}{42}=\frac{2871}{7}$
$=>b=594$
=>$\left \{ {a=1485} \atop {c=-792} \right.$
