\(\frac{12a-15b}{7}=\frac{20c-12a}{9}=\frac{15b-20c}{11}=\frac{12a-15b+20c-12a+15b-20c}{7+9+11}=0\)(tử bằng 0)

=> 12a - 15b = 20c - 12a = 15b - 20c => 12a = 15b = 20c

=>\(\frac{12a}{60}=\frac{15b}{60}=\frac{20c}{60}=\frac{a}{5}=\frac{b}{4}=\frac{c}{3}=\frac{a+b+c}{5+4+3}=\frac{48}{12}=4\)=> a = 4.5 = 20 ; b = 4.4 = 16 ; c = 4.3 = 12