Question

Cho tam giác ABC,vẽ 3 đường cao AD,BE,CF.CMR:

\(a)S_{AEF}=S_{ABC}.Cos^2A\\ b)AE.BF.CD=AB.AC.BC.CosA.CosB.CosC\\ c)\frac{S_{DEF}}{S_{ABC}}=1-\left(CosA-CosB-CosC\right)\)