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The statement in the title seems obviously true to me, but I can't quite prove it. Any suggestions would be appreciated.

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Hint $\ $ By Euclid's Lemma $\rm\ (b,a)=1,\,\ b\:|\:ak\:\Rightarrow\:b\:|\:k\:\Rightarrow\: b \le k < a$

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If $ka$ is a multiple of $b, ka=bc$ for some integer $c$.

$\implies c=\frac{ka}{b} \implies b\mid k$ as $(a,b)=1$

But as $a and $k can not divide $k$.

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    @DDD, ya, so $k$ can not be .2012-10-09