Citi’s operating expenses fell 18 percent to $10.7 billion as the bank, under Chief Executive Michael Corbat, put most of the problems stemming from the financial crisis behind it and exited businesses where profits and future prospects were not worthwhile. Citi shares rose 3 percent in premarket trading on Thursday. Total revenue in Citi’s core businesses, called Citicorp, declined 2 percent to $17.28 billion but the unit showed more efficiency, with operating expenses falling 18 percent.