连接OA,做OE垂直AB于E,OF⊥AC与F
∵OE⊥AB
∴AE=BE=√2/2
同理:
AF=CF=√3/2;
OA=1;
∴∠EAO=45°
∠FAO=30°
∠BAC=∠EA0+∠FAO=75°