The rate of change (t) (in thousands of articles per year) of the total number of research articles in a prominent journal written by researchers in the United States is shown in the following graph Here, t is time in years (t0 represents the start of 1983) and a1 4a 2a 0 24 6 8 10 12 14 16 18 20 22 (a) Use both left and right Riemann sums with six subdivisions to estimate the total number of articles in the ournal 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 0.25.) HINT [See Example 2. left sum right sum thousand articles thousand articles (b) Use the answers from part (a) to obtain an estimate ofr(t) dt. HINT [See Example 4. 10 Interpret the result. Since r(t) is given in thousands of articles per year, we conclude that a total of about in the journal were written by researchers in the U.S. in the articles year period beginning 1993