Tự vẽ hình nhá!
Ta có:
\(\dfrac{AC}{BC}=\dfrac{18}{27}=\dfrac{2}{3}\)
\(\dfrac{CD}{AC}=\dfrac{12}{18}=\dfrac{2}{3}\)
\(\Rightarrow\dfrac{AC}{BC}=\dfrac{CD}{AC}\)
Xét \(\Delta ADC\) và \(\Delta BAC\) có:
Góc C chung
\(\dfrac{AC}{BC}=\dfrac{CD}{AC}\)( cmt)
Do đó: \(\Delta ADC\sim\Delta BAC\) (c.g.c)
=> \(\dfrac{AD}{AB}=\dfrac{AC}{BC}\Rightarrow AD=\dfrac{AB.AC}{BC}=\dfrac{12.18}{27}=8\)
Vậy AD = 8(cm)
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