Quy nạp

K

kakashi_hatake

$4^n+15n-1 \ \vdots \ 9 \\ 4^{n+1}+15(n+1)-1=4.4^n+15n+14=4.(4^n+15n-1)-45n+18 \ \vdots \ 9 \rightarrow \text{đpcm} $
 
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