Giải thích các bước giải:
b.Ta có $DE//BC\to \widehat{DEC}=\widehat{ECB}=\widehat{ECA}=\widehat{ECD}$
$\to\Delta EDC$ cân tại $D$
$\to DE=DC$
Ta có:
$DE\cdot (\dfrac1{BC}+\dfrac{1}{AC})$
$=\dfrac{DE}{BC}+\dfrac{DE}{AC}$
$=\dfrac{DE}{BC}+\dfrac{DC}{AC}$
$=\dfrac{AD}{AC}+\dfrac{DC}{AC}$
$=\dfrac{AD+DC}{AC}$
$=\dfrac{AC}{AC}$
$=1$
$\to \dfrac1{BC}+\dfrac{1}{AC}=\dfrac{1}{DE}$