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若一個整數的數位和能被3整除,則這個整數能被3整除.若一個整數的個位數位截去,再從餘下的數中,减去個位數的2倍,如果差是7的倍數,則原數能被7整除.如果差太大或心算不易看出是否7的倍數,就需要繼續上述「截尾、倍大、…
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