The rate of change r(t) (in thousands of articles per year) of the total number of research articles in a prominent jaumai written by researchers in the United States is shown in the fallowing graph. Here, t is time in years (t = 0 represents the start of 1983). 20 15 The xf-coordinate plane is given. A curve passes through the rollowing approximate points. (0, 15.00) (1, 15.99) (2, 16.95) (3, 17.82) (4, 18.59) (5, 19.21) (6, 19.66) (7, 19.93) (a, 20.00) (9, 19.87) (10, 19.55) (11, 19.04) (12, 18.38) (13, 17.58) (14, 16.67) (15, 15.71) (16, 14.71) (17, 13.72) (18, 12.79) (19, 11.94) (20, 11.2z) (21, 10.54) (22, 10.24) (23, 10.03) (24, 10.02) (a) Use both left and right Riemann sums with six subdivisions to estimate the total number of articles (in thousands) in the journal written by researchers in the United States during the 12-year period beginning at the start of 1993. (Estimate each value of r(t) to the nearest 1.25.) left sum 250 thousand articles right sum 242.5 thousand articles (b) Use the answers from part (a) to obtain an estirmate of r(t) dt. 246.25 Interpret the result. Since r(t) is given in thousands articles per year, we conclude that a total of about 246,250 articles in the journal were written by rescarchers in the U.S. in the 12 -year period beginning 1993.