Bài 38 :
abcd = 1000a + 100b + 10c + d = ( 990a + 10a ) + ( 99b + b ) + 10c + d
= ( 990a + 99b ) + ( 10a + b + 10c + d )
= 11( 90a + 9b ) + ( ab + cd )
\(\text{Vì }\hept{\begin{cases}11⋮11\Rightarrow11(90a+9b)⋮11\\\text{ab + cd ⋮ 11 ( bài cho )}\end{cases}}\Rightarrow11(90a+9b)+ab+cd⋮11\)
=> abcd ⋮ 11
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