已知$f\left(x\right)=3\cos \left(x+\frac{3\mathrm{\pi }}{2}\right)+\cos \left(x-\frac{3\mathrm{\pi }}{2}\right)+\sin \left(x+\mathrm{\pi }\right)+a$（a∈R，a为常数）．

（1）若f（x）为奇函数，求a的值；

（2）若x∈R，求f（x）的最小正周期；

（3）若x∈0，$\frac{\mathrm{\pi }}{2}$时，f（x）的最大值为4，求a的值．

这篇小短文共____个自然段。

完形填空
      It's a fine Sunday morning. The weather is1. A lot of children2 in the park. They are3 happily. Some are playing4 under a big tree. Some girls are singing and dancing. Some boys are running5 the hill. Li Lei is6by the lake. He's reading a story. 7 is Wang Lin? He's standing over there. 8is he doing? He's looking9 a nice butterfly（蝴蝶）. He10to catch (捉住) it.