I guess formal and informal refer to two types of essay - you can write a formal essay where you avoid conversational language and such and where you need to be very professional, or you can write informally where you don't have to think about such things, it is very casual.
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Answer:
1. infamous (a mobster would be considered infamous because he is a bad person, but is well known; such as Al Capone)
2. true
3. opinion
4. I can't answer this because I dont have access to your class notes BUT my guess would be: 1, 2, 4, 5
Explanation: