Tham khảo
`a) 3^{n+2}-3^{n+1}-6.3^n`
`=3^n×9-3^n×3-3^n.6`
`=3^n.(9-3-6)`
`=3^n.0`
`=0`
`b) (3.2^{n-2}+2^n-2^{n-1}):5`
`=(3.2^n:4+2^n-2^n:2):5`
`=(2^n.\frac{3}{4}+2^n-2^n.\frac{1}{2}):5`
`=[2^n.(\frac{3}{4}+1-\frac{1}{2})]:5`
`=[2^n:\frac{5}{4}]:5`
`=\frac{2^{n+2}}{5}:5`
`=\frac{2^{n+2}}{25}`
`c) 10^{n+1}-6.10^n`
`=10^n.10-6.10^n`
`=10^n.(10-6)`
`=10^n.4`
`d) 2^{n+3}+n^{n+2}-n^{n+3}+2^n`
`=2^n.8+n^n.n^2-n^n.n^3+2^n`
`=(2^n.8+2^n).(n^n.n^2-n^n.n^3)`
`=2^n.(8+1+n^n.(n^2-n^3)`
`=2^n.9+n^n.[n^2.(1-n)]`
`e) 90.10^n-10^{n+2}+10^{n+1}`
`=9.10^{n+1}-10^{n+1}.10+10^{n+1}`
`=10^{n+1}.(9-10+1)`
`=10^{n+1}.0`
`=0`
$\text{©CBT}$
