a) \(7^6+7^5-7^4=7^4\left(7^2+7-1\right)=7^4.55⋮55\)
d) \(81^7-27^9-9^{13}\)
\(=\left(3^4\right)^7-\left(3^3\right)^9-\left(3^2\right)^{13}\)
\(=3^{28}-3^{27}-3^{26}\)
\(=3^{26}\left(3^2-3-1\right)=3^{226}.5=3^{222}.405⋮405\)
c) Nhóm 4 hạng tử lại sẽ ra vầy
\(120\left(1+3^5+...+3^{95}\right)⋮120\)
b) \(=3^n\left(3+27\right)+5^n\left(25+125\right)\)
\(=3^n.30+5^n.150=30\left(3^n+5^n.5\right)⋮30\)
Câu b là vầy :
( 3n+3 + 3n+1 ) + ( 5n+3 + 5n+2 )
= 3n. 33 + 3n + 3 + 5n. 53 + 5n. 52
= 3n . ( 27 + 3 ) + 5n . (25 + 125)
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