y'=-2x设点P为(a,1-a^2)则切线为y-1+a^2=(-2a)(x-a)=2a^2-2ax等价于y+2ax=a^2+1与x轴y轴的交点为(a^2+1/2a,0)和(0,a^2+1)所围的三角形的面积为(a^2+1)^2/(|4a|)=(a^2+1/3+1/3+1/3)^2/(|4a|)>=4/9根号3当且仅当a=根号...