cho (o) , duong kinh AC,B ∈ (o)sao cho sd b∧c =60.Qua B ke BD ⊥AC. QUA D ke DF//AC. tinh sd cung CD,AB , FD , AF ,

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1. Ta có:

   $+)\quad BD\perp AC$

   $\to sđ\mathop{CD}\limits^{\displaystyle\frown}=sđ\mathop{BC}\limits^{\displaystyle\frown} = 60^\circ$

   $+)\quad sđ\mathop{AB}\limits^{\displaystyle\frown} = sđ\mathop{AC}\limits^{\displaystyle\frown} – sđ\mathop{BC}\limits^{\displaystyle\frown}$

   $\to sđ\mathop{AB}\limits^{\displaystyle\frown} = 180^\circ – 60^\circ = 120^\circ$

   $+)\quad DF//AC$

   $\to sđ\mathop{AF}\limits^{\displaystyle\frown}=sđ\mathop{CD}\limits^{\displaystyle\frown} = 60^\circ$

   $+)\quad sđ\mathop{FD}\limits^{\displaystyle\frown} = sđ\mathop{AC}\limits^{\displaystyle\frown} – (sđ\mathop{CD}\limits^{\displaystyle\frown}+ sđ\mathop{AF}\limits^{\displaystyle\frown})$

   $\to sđ\mathop{FD}\limits^{\displaystyle\frown} = 180^\circ – 2.60^\circ = 60^\circ$

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