O为X轴Y轴的交点,设角ACO=α, 角BCO=βtanACB=tan(β-α)=(tanβ- tanα)/(1+ tanβ* tanα)=2x/(x^2+2)=2/(x+2/x)≤1/√2x=±√2c点为(-√2,0)或者(√2,0)