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`b.`

`B=(5.4^15 .9^9 -4.3^20 .8^9)/(5.2^10 .6^19 -7.2^29 .27^6)`

`=(5.(2^2)^15 .(3^2)^9 -2^2 .3^20 .(2^3)^9)/(5.2^10 .(2.3)^19 -7.2^29 .(3^3)^6)`

`=(5.2^30 .3^18 -2^2 .3^20 .2^27)/(5.2^10 .2^19 .3^19 -7.2^29 .3^18)`

`=(5 .2^30 .3^18 -2^29 .3^20)/(5.2^29 .3^19 -7.2^29 .3^18)`

`=(2^29 .3^18 (5.2-3^2))/(2^29 .3^18 (5.3-7))`

`=(5.2 -3^2)/(5.3-7)`

`=(10-9)/(15-7)`

`=(1)/(8)`

Vậy `B=(1)/(8)`