如图,在△ABC中,AD是高,DE ⊥ AB ,DF ⊥ AC ,垂足分别为 E ,F ,求证:∠B=∠AFE.
如图,在△ABC中,AD是高,DE ⊥ AB ,DF ⊥ AC ,垂足分别为 E ,F ,求证:∠B=∠AFE.
证明;
∵DE⊥AB, DF⊥AC, AD⊥BC
∴∠DAF+∠C=90度,∠DAF+∠ADF=90度,
∠DAE+∠B+90度,∠DAE+∠ADE=90度,
∴∠ADF=∠C, ∠ADE=∠B
又∵∠DAF=∠DAF ∠ADC=∠AFD=90度,
∠DAE=∠DAE ∠ADB=∠AED=90度
∴△AFD∽△ADC △ADE∽△ADB
∴AF/AD=AD/AC, AE/AD=AD/AB
即;AD^2=AF*AC, AD^2=AE*AB
即;AF*AC=AE*AB
即;AF/AB=AE/AC
又 ∵∠ FAE=∠BAC
∴△AFE∽△ABC,
∴∠B=∠AFE