The idea of a large, stable middle class (defined as those with annual household incomes in 2010 between $42,000 and $126,000 for a family of three), is central to america's sense of itself. but the u. s. middle class shrank steadily from 61% of all adults in 1971 (t = 0) to 51% in 2011 (t = 4), where t is measured in decades. research has shown that this decline was approximately linear.† (a) find a linear function f(t) giving the percentage of middle-income adults in decade t, where t = 0 corresponds to 1971.