2, DE= root number 2, invariant (rectangular fixed point); OE= 1, unchanged (the median line on the hypotenuse of a right triangle is equal to half of the hypotenuse, the right angle is unchanged, and the hypotenuse is unchanged, so the length of this median line is unchanged);

3. Connecting OD, we know that OE+DE is greater than or equal to OD (any two sides of a triangle are greater than the third side), that is, the root number 2+ 1 is greater than or equal to OD, so it is the largest when OD is equal to the root number 2+ 1 (at this time, O, D and E are on the same line).

I don't know. You got it? I wish you good grades!