xét \(\Delta ACU\) với \(4BCK\)
\(\widehat{C}\) chung
\(\widehat{AUC}=\widehat{BKC}=90^0\)
\(\Rightarrow\widehat{CAU}\infty\Delta CBK\)
\(\Rightarrow\dfrac{CU}{CK}=\dfrac{AU}{BK}\)
\(\Rightarrow CK.AU=AU.BK\)
\(\Rightarrow CK^2.AU^2=CU^2.BK^2=\dfrac{BC^2}{4}.BK^2\)
\(CK^2=BC^2-BK^2\)
\(\Rightarrow\left(BC^2-BK^2\right)AU^2=\dfrac{BC^2.BK^2}{4}\)
chưa có \(2n\) cho \(AU^2.BC^2.BK^2\)
\(\dfrac{1}{BK^2}=\dfrac{1}{BC^2}+\dfrac{1}{BD^2}=\dfrac{1}{BC^2}+\dfrac{1}{4AH^2}\left(đpcm\right)\)
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