`a)2^9-1=(2^3-1)[(2^3)^2+2^3+1]=(2^3-1).73 \vdots 73`
`=>Đpcm`
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`b)5^6-10^4=5^6-2^4 .5^4=5^4 .(5^2-2^4)=5^4 .9 \vdots 9`
`=>Đpcm`
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`c)(n+3)^2-(n-1)^2=(n+3-n+1)(n+3+n-1)=4(2n+2)=8(n+1) \vdots 8 AA n in N`
`=>Đpcm`
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`d)(n+6)^2-(n-6)^2=(n+6-n+6)(n+6+n-6)=12.2n=24n \vdots 24 AA n in N`
`=>Đpcm`
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