The answer to exercise marked [BB] can be found in the Back of the Book.
Prove that an integer (an−1 an−2 … a0)10 is divisible by 11 if and only if a0 + a2 + a4 + … ≡ a1, + a3 + a5 + … (mod 11). [Hint. 10 ≡ −1 (mod 11).]
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