17. Finance. Please provide complete and correct solution done on computer or by hand with mathematical proof/explanation to all questions. Please, emphasis on complete and correct solution. The answers will be verified. This is all the information provided. Thank you very much.

Please provide all sources and assumptions for these questions.

Many companies in the U.S. increased their debt from circa 1985 to circa 2009. a) Provide reasons why they did so. b) In 2009, what happened to companies in the U.S. that had increased their debt substantially?