Giúp mình với ạ cảm ơn

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Giúp mình với ạ cảm ơn

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    Giải thích các bước giải:

    a.Xét $\Delta ABM,\Delta ACN$ có:

    $BA=AC$

    Chung $\hat A$

    $MA=AN$

    $\to \Delta ABM=\Delta ACN(c.g.c)$

    b.Từ câu a $\to BM=CN$
    Xét $\Delta BNC, \Delta BCM$ có:

    Chung $BC$

    $CN=BM$

    $NB=AB-AN=AC-AM=MC$

    $\to \Delta BNC=\Delta CMB(c.c.c)$

    c.Ta có: $\Delta ABC$ cân tại $A$

    $\to\widehat{BAC}+2\widehat{ABC}=180^o$

    $\to \widehat{BAC}+\widehat{BAC}=180^o$

    $\to2\widehat{BAC}=180^o$

    $\to \widehat{BAC}=90^o$

    $\to \widehat{ABC}=\widehat{ACB}=\dfrac12\widehat{BAC}=45^o$

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