x=y+根号2
代入x的平方+y的平方=25
(y+√2)^2+y^2=25
2y^2+2√2y-23=0
设交点为(x1,y1) (x2,y2)则
y1+y2=-√2 y1y2=-23/2
(y1-y2)^2=(y1+y2)^2-4y1y2=2+4*23/2=48
又x1-x2=(y1+√2)-(y2+√2)=y1-y2
所以弦长=√[(x1-x2)^2+(y1-y2)^2]=√[2(y1-y)^2]=√(2*48)=4√6