—It's a long time ________ I saw you last.

—Yes，and what a pity！It will be a long time ________ we see each other again.

声音在空气中传播的速度大约是　 ____　米/秒．人耳能分清前后的两个声音的时间间隔是大于0.1秒．要想听见回声，人离障碍物的距离应大于　 ____　米．

My brother's birthday is on  ____(twenty-one)

①∠AGD=112.5°；②tan∠AED= $\sqrt{2}$ +1；③四边形AEFG是菱形；④S_△ACD= $\sqrt{3}$ S_△OCD．

其中正确结论的序号是____．（把所有正确结论的序号都填在横线上）