如图，平行的无限长直载流导线A和B，电流强度均为I，垂直纸面向外，两根载流导线之间相距为a，则<br /> （1）<img src="https://nimg.ppkao.com/2019-04/wangjue/2019041515490839084.jpg?sign=d77cef8fdd79e65d6500aaa2355ba494&t=62d5c88c" />中点（P点）的磁感强度<img src="https://nimg.ppkao.com/2019-04/wangjue/2019041515492580606.jpg?sign=f5acbc926d4dbbcfe32d02887bbed6f4&t=62d5c88c" />=（）。<br /> （2）磁感强度<img src="https://nimg.ppkao.com/2019-04/wangjue/2019041515494228668.jpg?sign=5e47bce66bb0a2cc45cdb9bd4dfc5292&t=62d5c88c" />沿图中环路L的线积分<img src="https://nimg.ppkao.com/2019-04/wangjue/2019041515495769266.jpg?sign=a19a254b975c763687a8cd6ddd9b276e&t=62d5c88c" />=（）。<br /> <img src="https://nimg.ppkao.com/2019-04/wangjue/201904151550124014.jpg?sign=c7120e1193655afb2ea47297f47a86af&t=62d5c88c" />