△OEF，OE=OF=√2，EF=2。

∵(√2)? +(√2)? =2?

∴OE？ +OF？ =EF？

That is, △OEF is Rt△, ∠ EOF = 90.

∫ Point E is the midpoint of arc AC, and point F is the midpoint of arc AB.

∴ arc BC is: 90× 2 = 180.

BC is the diameter of this circle.

∴∠CAB=90

Namely: AC⊥AB