∫OD≤OE+DE,
∴ When three straight lines O, D and E form * * *, the distance from point D to point O is the largest.
At this time, AB = 2, BC= 1,
∴OE=AE= 12AB= 1,
DE= AD2+AE2= root number = 1 below? + 1? ) = 2 under the root sign,
∴ The maximum value of OD is: 2+ under the root sign? 1.