设点是(a,b)在y=x²上b=a²(a,a²)到x-y-2=0距离=|a-a²-2|/√(1²+1²)=|a²-a+2|/√2=|(a-1/2)²+7/4|/√2分子最小值=7/4所以最短距离=(7/4)/√2=7√2/8