a=(1/3+1/9)÷2/3=4/9÷2/3=4/9×3/2=2/3
b=5/6÷21/9×(1+1/2×3/9)=5/6÷21/9×(1+3/18)=5/6×9/21×21/18=5/12
c=9/3-(1/3-(1/3-3/2×(1/3-1/4)))=9/3-(1/3-(1/3-3/2×1/12)=9/3-(1/3-(1/3-3/24))=9/3-(1/3-5/24)=9/3-3/24=3-1/8=2,7/8
