Sets P1,P2,S1,S2 | (P2 intersection S1) C P1,(P1 intersection S2) C P2 and (S1 intersection S2) C (P1 U P2),Demonstrate that (S1 intersection S2) C (P1 intersection P2)
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We are given sets `P_1, P_2, S_2, S_2` and know that:
1. `P_2 nnS_1 \subset P_1`
2. `P_1 nn S_2 \subset P_2`
3. `S_1 nn S_2 \subset P_1 uu P_2`
We must show that `S_1 nn S_2 \subset P1 nn P_2`
(In English: show every everything in both S1 and S2 is also in both P1 and P2)
Let `x` be an element of `S_1 nn S_2` . This implies that `x in S_1` and `x in S_2` . By (3), we also know that either `x in P_1` or `x in P_2` .
Case one: suppose `x in P_1` : By (2), we see `x in S_2 ` and `x in P_1` and by (2) `P_1 nn S_2 \subset P_2` , therefore `x in P_2` .
Case two: suppose` x in P_2` . Since` x in S_1` and `P_2 nn S_1 \subset P_1` , it follows that `x in P_1` .
Since each of these cases shows x must exist in both P1 and P2, it follows that `S_1 nn S_2 \subset P_1 nn P_2`
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