Bài 1:
a) \(7^6+7^5-7^4\)
\(=7^4.7^2+7^4.7-7^4\)
\(=7^4.\left(7^2+7-1\right)\)
\(=7^4.\left(49+7-1\right)\)
\(=7^4.55\)
\(=7^4.5.11\)
Vì \(11⋮11\) nên \(7^4.5.11⋮11\)
\(\Rightarrow7^6+7^5-7^4⋮11.\)
b) \(81^7-27^9-9^{13}\)
\(=\left(3^4\right)^7-\left(3^3\right)^9-\left(3^2\right)^{13}\)
\(=3^{28}-3^{27}-3^{26}\)
\(=3^{26}.\left(3^2-3-1\right)\)
\(=3^{26}.5\)
\(=3^{13}.3^2.5\)
\(=3^{13}.45\)
Vì \(45⋮45\) nên \(3^{13}.45⋮45\)
\(\Rightarrow81^7-27^9-9^{13}⋮45.\)
Chúc bạn học tốt!
a) 7^6+7^5-7^4 chia hết cho 11
= 7^4 ( 7^2 + 7 - 1 )
= 7^4 ( 49 +7 - 1 )
= 7^4 + 55
= 7^4 x 5 x 11 chia hết cho 11 ( đpcm )
b ) 81^7-27^9-9^13 chia hết cho 45
= (3^4)^7 - ( 3^3)^9 - ( 3^2 )^13
= 3^ 28- 3 ^27 - 3^26
= 3 ^26 x ( 3^2 - 3^1 - 3^0 )
= 3^24 x 9 x5
= 3 ^24 x 45 chia hết cho 45 ( đpcm )
