Xét \(∆ ADC\) và \(∆ BEC\) có:
+) \(\widehat {ADC} = \widehat {BEC} = 90^\circ \)
+) \(\widehat C\) chung
\( \Rightarrow ∆ ADC \backsim ∆ BEC\) (g.g)
\( \Rightarrow\displaystyle{{AC} \over {BC}} = {{DC} \over {EC}}\)
\( \Rightarrow\displaystyle {{EC} \over {BC}} = {{DC} \over {AC}}\)
Xét \(∆ DEC\) và \(∆ ABC\) có:
+) \(\displaystyle {{EC} \over {BC}} = {{DC} \over {AC}}\)
+) \(\widehat C\) chung
\( \Rightarrow ∆ DEC\backsim ∆ ABC\) (c.g.c)
