The following table gives the the approximate amount of emissions, E. of nitrogen oxides in millions of metric tons per year in the US. Let t be the number of years since 1940 and E = f(t). a. Using left endpoints, estimate the integral: 50 f(t) dt = b. Using right endpoints, estimate the integral: 50 f(t) dt Be sure that you know what the units of your answer are, and what its meaning is! (Original data from the Statistical Abstract of the US, 1992) t 1940 1950 1960 1970 1980 1990 E 7.4 9.3 13.3 18.1 21.3 19.6