Explanation:
Given the equation that modeled the total public debt D (in trillions of dollars) in the United States at the beginning of each year from 2000 through 2008 can be approximated by;
D = 0.032t^2 + 0.21t + 5.6, 0 ≤ t ≤ 8
We are to find the total public debt with the time interval;
when t = 0;
D = 0.032(0)^2 + 0.21(0) + 5.6
D = 5.6 trillion dollars
when t = 1;
D = 0.032(1)^2 + 0.21(1) + 5.6
D = 0.032+0.21+5.6
D = 5.842 trillion dollars
when t = 2;
D = 0.032(2)^2 + 0.21(2) + 5.6
D = 0.128+0.42+5.6
D = 6.148 trillion dollars
when t = 3;
D = 0.032(3)^2 + 0.21(3) + 5.6
D = 0.288+0.63+5.6
D = 6.518 trillion dollars
when t = 4;
D = 0.032(4)^2 + 0.21(4) + 5.6
D = 0.512+0.84+5.6
D = 6.952 trillion dollars
when t = 5;
D = 0.032(5)^2 + 0.21(5) + 5.6
D = 0.8+1.05+5.6
D = 7.45 trillion dollars
when t = 6;
D = 0.032(6)^2 + 0.21(6) + 5.6
D = 1.152+1.26+5.6
D = 8.012 trillion dollars
when t = 7;
D = 0.032(7)^2 + 0.21(7) + 5.6
D = 1.568+1.47+5.6
D = 8.638 trillion dollars
when t = 8;
D = 0.032(8)^2 + 0.21(8) + 5.6
D = 0.032(64)+1.68+5.6
D = 2.048+1.68+5.6
D = 9.328 trillion dollars
From the values gotten, we can see that the total public debt reached or surpassed 7 trillion dollars in 2005
To get the public debt in 2017, we can simply substitute t = 17 into the expression D = 0.032t2 + 0.21t + 5.6
D = 0.032(17)^2 + 0.21(17) + 5.6
D = 0.032(289)+3.57+5.6
D = 9.248+3.57+5.6
D(17) = 18.418 trillion dollars