分解因式：-$\frac{1}{2}$x⁴+8=____ ；（a²+1）²﹣4a²=____ 　．

如图，为测得河对岸塔AB的高，先在河岸上选一点C，使C在塔底B的正东方向上，测得点A的仰角为60°，再由点C沿北偏东15°方向走10m到位置D，测得∠BDC=45°，则塔AB的高是（　　）（单位：m）

下列函数中，自变量$x$的取值范围是$x\ge 2$的是（    ）

听短文，写出所缺部分。

    Hi! My name is Linda! I'm a middle school student. It was____lastSunday．After breakfast, my grandparents went for a walk in the park with their dog．They liked the____air there．My father____with his friend．My mother had to____．What about my brother? He read a book about____in his room．I practiced the piano in the living room．

分别用分度值为1m、1dm、1cm、1mm四种直尺，测量同一物体的长度，并用米为单位作记录，则记录数据中小数点后面的位数最多的是（　　）