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) .

1940 | 1950 | 1960 | 1970 | 1980 | 1990 | |

6.6 | 9.5 | 12.7 | 18.8 | 21.4 | 19.6 |

(a) Using left endpoints, estimate the integral:

(b) Using right endpoints, estimate the integral:

*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)*

You can earn partial credit on this problem.