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$\begingroup$

I can not understand that how it is proved, so please somebody help me. I unable to prove but I think you will able to prove it.

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    One way is to show that $Ha\to a^{-1}H$ is a bijection (or for this just injection is enough) between left and right cosets2017-02-14
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    Please prove in details.2017-02-14
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    is this about groups?2017-02-14
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    Yes this is about group2017-02-14

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As $Ha,Hb$ are equivalence classes, $Ha\neq Hb \iff a\notin Hb\iff ab^{-1}\notin H\iff (ab^{-1})^{-1}=ba^{-1}\notin H\iff a^{-1}\notin b^{-1}H\iff a^{-1}H\neq b^{-1}H$