F1F2=2,c=1 F1(-1,0) F2(1,0) P(1,3|2) |PF1|+|PF2|=2a a=2 b^2=3 椭圆方程x^2/4+y^2/3=1
直线L:x=my-1 A(x1,y1),B(x2,y2) SAF2B=1/2*|F1F2|*|y2-y1|=|y2-y1|
联立x^2/4+y^2/3=1和x=my-1 (3m^2+4) y^2-6my-9=0 |y2-y1| =根号[(y1+y2)^2-4y1y2]
=根号[(144m^2+144)/(3m^2+4)]=12根号2/7 m=-1,m=1
直线L:x=-y-1 x=y-1
d=r=根号2 圆的方程(x-1)^2+y^2=2