联立解方程得x1+x2=2b+2 x1x2=b^2
垂直的x1x2=-y1y2
y1^2=2x1
y2^2=2x2
所以 x1x2=4=b^2 b=2
x1+x2=6
x1x2=4
s=(oa+ob)/2=3√2