简述《三国演义》中的一次战役。
参考答案：
①官渡之战
②赤壁之战
③彝陵之战
④水淹七军
⑤汉中争夺战
若（a-上）2+|b-2|=0，则以a，b为直角边的直角三角形的斜边长为______．
Why is it important to have a project charter ？（）
A．It is the collection of all subsidiary project documents.
B．The project charter is the necessary basis for all work packages to be performed.
C．It formally authorizes the project.
D．It tracks the progress of the project.
