1.若$P_1,P_2$为数域,证明:$P_1 \bigcap P_2$也为数域. 证:设$P_1\bigcap P_2=P$, $\because P_1,P_2$是数域,$\therefore 0,1\in P_1$,$0,1\in P_2$ $\therefore 0,1 \in P$ 设$\forall a,b\in P$,由题$a,b\in P_1$且$a,b\in P_2$ $\because P_1,P_2$是数域,$\therefore a+b\in P_1 , a+b \in P_2$ $\the…