Tính nhanh: 20/11 x 33/23 x 69/180

`\frac{20}{11} xx \frac{33}{23} xx \frac{69}{180} `

`= \frac{20xx33xx69}{11xx23xx180} `

`= \frac{20 xx 11 xx3 xx 3xx23}{11 xx23xx20xx9} `

`= \frac{20xx11xx9xx23}{11 xx23xx20xx9} `

`= 1`

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