A = 3 + 32 + 33 + ... + 39 + 310
=> A = ( 3 + 32 ) + ( 33 + 34 ) + ... + ( 39 + 310 )
=> A = 3( 1 + 3 ) + 33( 1 + 3 ) + ... + 39( 1 + 3 )
=> A = 3 . 4 + 33 . 4 + ... + 39 . 4
=> A ( 3 + 33 + ... + 39 ).4 chia hết cho 4
Vậy A chia hết cho 4
A = 3 + 32 + 33 + ... + 39 + 310
=> A = ( 3 + 32 ) + ( 33 + 34 ) + ... + ( 39 + 310 )
=> A = 3( 1 + 3 ) + 33( 1 + 3 ) + ... + 39( 1 + 3 )
=> A = 3 . 4 + 33 . 4 + ... + 39 . 4
=> A = 4 ( 3 + 33 + ... + 39 )
4 chia hết cho 4 => A chia hết cho 4
\(A=3+3^2+3^3+...+3^9+3^{10}\)
\(A=\left(3+3^2\right)+\left(3^3+3^4\right)+...+\left(3^9+3^{10}\right)\)
\(A=3.\left(1+3\right)+3^3.\left(1+3\right)+...+3^9.\left(1+3\right)\)
\(A=3.4+3^3.4+...+3^9.4\)
\(A=4.\left(3+3^3+3^9\right)\)
