de thi thu vao 10

I

innocent_baby

P

pe_lun_hp

Tớ làm câu 3,4 thôi nhé
:)
3.
$cos A = \dfrac{AD}{AB} = \dfrac{AE}{AC}$

\Rightarrow $\Delta{EAD} \sim \Delta{CAB}$

\Rightarrow $\dfrac{DE}{BC} = \dfrac{AD}{AB} = cos A$

4.
CM : $\hat{OAD} +\hat{ADE}=90^o$
 
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