P(m,2)
AB
xA+xB=2m,yA+yB=4
(yA)^2+(yB)^2=4(xA+xB)
(yA+yB)^2-2yA*yB=4*2m
4^2-2yA*yB=8m
yA*yB=8-4m
(yA-yB)^2=(yA+yB)^2-4yA*yB=16-4*(8-4m)=16m-16
(yA)^2-(yB)^2=4(xA-xB)
(yA+yB)*(yA-yB)=4(xA-xB)
4*√(16m-16)=4(xA-xB)
(xA-xB)^2=16m-16
AB^2=(xA-xB)^2+(yA-yB)^2=32m-32
m无穷大,弦AB长的最大值=无穷大