Question

Cho tam giác ABC ,góc BAC =120^o,AD là p/g

CMR 1/AB=1/AC=1/AD

Answer 1

Kẻ \(DE||AB\left(E\in AC\right)\)

\(\Rightarrow\frac{DE}{AB}=\frac{EC}{AC}\)

\(\Delta ADE\)đều (vì.............)\(\Rightarrow AD=AE=DE\)

\(\Rightarrow\frac{AD}{AB}=\frac{AC-AE}{AC}\)mà \(AE=AD\)

\(\Rightarrow\frac{AD}{AB}=1-\frac{AD}{AC}\)

\(\Rightarrow\frac{AD}{AB}+\frac{AD}{AC}=1\)

\(\Rightarrow AD\left(\frac{1}{AB}+\frac{1}{AC}\right)=1\)

\(\Rightarrow\frac{1}{AB}+\frac{1}{AC}=\frac{1}{AD}\left(ĐPCM\right)\)