I'm horrible at proofs. Can someone help me with this problem? Many thanks

Show that the postulates of probability are satisfied by conditional probabilities. IN other words, show that if P(B)≠0

1. P(A|B)≥0

2. P(B|B)=0

3. P(A1UA2U...|B)=P(A1|B)+P(A2|B)+...for any sequence of mutually exclusive events A1, A2...