Use indirect proof to show that the argument is valid.
1. Sam goes to the store only if he needs milk. Sam does not need milk. Therefore, Sam does not go to the store.
2. If it rains hard, there will be no picnic. If Dave brings the Frisbee, the kids will be happy. The kids are not happy and there is a picnic. Therefore, it does not rain hard and Dave did not bring the Frisbee.
3. If the newspaper and television both report a crime, then it is a serious crime. If a person was killed, then the newspaper reports the crime. A person is killed. Television reports the crime. Therefore, the crime is serious.
4. If Linda feels ill, she takes aspirin. If she runs a fever, she does not take a bath. If Linda does not feel ill, she takes a bath. Linda runs a fever. Therefore, she takes aspirin?