Vì ABC là tam giác cân
\(\Rightarrow\widehat{B}=\widehat{C}=\dfrac{180^o-70^o}{2}=55^o\)
Xét tứ giác ABDH có:
\(\widehat{A}+\widehat{B}+\widehat{AHD}+\widehat{BDH}=360^o\)
\(\Rightarrow\widehat{BDH}=360^o-70^o-55^0-90^o=145^o\)
\(b,\) Ta có:
\(\widehat{BDH}+\widehat{HDC}=180^o\)
\(\Rightarrow\widehat{HDC}=180^o-145^o=35^o\)
Mà \(\widehat{A}=70^o\)
\(\Rightarrow\widehat{A}=2\widehat{HDC}\)
=> đpcm
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