`9_(10)=?_2`
`9 div 2 = 4 \text{ dư 1}`
`4 div 2 = 2 \text{ dư 0}`
`2 div 2 = 1 \text{ dư 0}`
`1 div 2 = 0 \text{ dư 1}`
`=>9_(10)=1001_2`
`12_(10)=?_2`
`12 div 2 = 6 \text{ dư 0}`
`6 div 2 = 3 \text{ dư 0}`
`3 div 2 = 1 \text{ dư 1}`
`1 div 2 = 0 \text{ dư 1}`
`=>12_(10)=1100_2`
`15_(10)=?_2`
`15 div 2 = 7 \text{ dư 1}`
`7 div 2 = 3 \text{ dư 1}`
`3 div 2 = 1 \text{ dư 1}`
`1 div 2 = 0 \text{ dư 1}`
`=>15_(10)=1111_2`
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