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Ta có:

`81 = 3^4`

`15^2 = 3^2xx5^2`

`675 = 3^3xx5^2`

`10^4 = 2^4xx5^4`

`4^4 = 2^8`

 `-` Suy ra:

`A = (2^4xx5^4xx3^4 − 2^4xx3^2xx5^2) / (2^8xx3^3xx5^2)`

`= [2^4xx3^2xx5^2(3^2xx5^2 − 1)] / (2^8xx3^3xx5^2)`

`= [2^4xx3^2xx5^2xx224] / (2^8xx3^3xx5^2)`

`= 224 / (2^4xx3)`

`= 224 / (16xx3)`

`= 224 / 48`

`= 14 / 3`

Vậy `A = 14 / 3.`