令t= √(1-2x),则t>=0y=x- √(1-2x)=(-1/2)(1-2x)- √(1-2x)+1/2=(-1/2)t^2-t+1/2=(-1/2)(t^2+2t-1)=(-1/2)[(t+1)^2-2]=(-1/2)(t+1)^2+1t>=0 (t+1)^2>=11-(1/2)(t+1)^2