Ta có : 87 = ( 23) 7 = 221
=> 221 - 218 = 218. ( 23 - 1 ) = 218 . 7 = 217 .2 . 7 = 217 . 14
Vì 14 chia hết cho 14
=> 217 .14 chia hết cho 14
Vậy 87 - 218 chia hết cho 14
\(8^7-2^{18}=2^{21}-2^{18}\)
\(=2^{18}\left(2^3-1\right)\)
\(=2^{18}.7\)
\(=2^{17}.2.7\)
\(=2^{17}.14⋮14\left(ĐPCM\right)\)
87 _ 218 =(23)7-218=221-218 =218.23-218.1=218.(23-1) =218.7=217.(2.7)=217.14
=>217chia hết cho 14=>87 _ 218 chia hết cho 14
\(8^7-2^{18}=\left(2^3\right)^7-2^{18}\)
\(=2^{21}-2^{18}=2^{18}.\left(2^3-1\right)\)
\(=2^{18}.7=2^{17}.2.7=2^{17}.1\text{4}⋮1\text{4}\left(Đ\text{p}CM\right)\)
