Since H is orthocentre of △ABC, ∠AOC=90° ∠BCS=90° (∠ in semi-circle) ∠AOC+∠BCS=180° ∴AH//SC
Produce CH and meet at AB. Let Z be their intersection. Since H is orthocentre of △ABC, ∠AZC=90° ∠AZC=∠SCB ∠ZAC=∠CSB (∠s in same seg.) ∴△AZC~△SCA (AA)
∠ACZ=∠SBC (corr. ∠s, ~△s) ∠SAC=∠SBC (∠s in same seg.) ∠SAC=∠ACZ ∴AS//HC Since AS//HC and AH//SC, AHCS is a //gram.作者: 抹茶可樂 時間: 13/7/2006 04:00 AM 呢條咪係(..........)我在戰場做果條