-> 矢印

(AP1)^2=1+(P1-1)^2
(AP2)^2=1+(P2-1)^2
y=-(P2/P1)(x-1)+1
P2=(P2/P1)+1
(P1-1)P2=P1
P2=P1/(P1-1)
(P2-1)^2=1/(P1-1)^2
(AP1)^-2+(AP2)^-2=1
(AP)^2=1