Cho tam giác ABC vuong tai A .Duong cao AH. Goi D, E lan luot la hinh chieu cua H tren AB va AC
a) AH = DE
HD//EA; HE//DA ;^A=90° =>ADHE la hinh chu nhat
AH, DE la hai duong cheo => AH=DE
b) Goi M,N lan luot la trung diem cua BH va CH. C/m Smnde=1/2 Sabc.
(kqa)=>∆DEH=∆DEA
M, trung diem BH =>S∆dbm=S∆dmh
N, trung diem CH =>S∆ehn=S∆ehc
Smnde=S∆deh+S∆dmh+S∆ehn
S∆abc=2.(S∆deh+S∆dmh+S∆ehn)=2S.mnde =>dpcm