20 12 Jinan senior high school entrance examination mathematics 13 titled "What is the midpoint between A and B?
It is because the length range of OD is transformed and put into triangle ODE to solve the value range of OD length. DE= root number 2, OE= 1, the sum of any two sides of the triangle is greater than the third side, and root number 2+ 1 is greater than or equal to OD, so OD is the largest when it is equal to root number 2+ 1.