Hello! Look here! There's 1 apple tree behind Dick's house. Dick is near the tree. There are some apples 2 the tree. Dick is looking 3the apples. They are big and red. Dick is hungry. He wants to 4 an apple. Dick isn't tall. He is short. He can't reach(够到) the apples. He puts a 5 under the tree. Can he reach the apples on the box now?6, he can't. He is too7. Here comes Paul. He is Dick's brother. “Hello, Dick,8 do you want to do?”

     “Please help9, Paul. I can't 10 the apples.”

     “Here you are, Dick. One is for you, and one is for me!”

     “Thank you, Paul.”

如图1，⊙O的半径为r（r＞0），若点P′在射线OP上，满足OP′•OP=r²，则称点P′是点P关于⊙O的“美好点”．如图2，⊙O的半径为2，点B在⊙O上，∠BOA=60°，OA=4，若点A′、B′分别是点A，B关于⊙O的美好点，求A′B′的长．