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Bài 3: a> Ta có $\frac{12}{9}=\frac{8}{x}$ $\frac{12}{9}=\frac{8}{x}$ Suy ra $12\times x=9\times 8$ $12\times x=72$ $x=72:12$ $x=6$ Ta có $\frac{12}{9}=\frac{y}{21}$ Suy ra $12\times 21=9\times y$ $252=9\times y$ $y=252:9$ $y=28$ Vậy $A=y-x=28-6=22$ b> Ta có $\frac{7}{6}=\frac{x}{18}$ Suy ra $7\times 18=6\times x$ $126=6\times x$ $x=126:6$ $x=21$ Ta có $\frac{7}{6}=\frac{98}{y}$ Suy ra $7\times y=6\times 98$ $7\times y=588$ $y=588:7$ $y=84$ Ta có $\frac{7}{6}=\frac{14}{z}$ Suy ra $7\times z=6\times 14$ $7\times z=84$ $z=84:7$ $z=12$ Vậy $B=x+y+z=21+84+12=117$