#### Problem 25E

25. The table gives the population of the world $P(t)$, in millions, where $t$ is measured in years and $t=0$ corresponds to the year 1900 .

(a) Estimate the rate of population growth in 1920 and in 1980 by averaging the slopes of two secant lines.

(b) Use a graphing device to find a cubic function (a thirddegree polynomial) that models the data.

(c) Use your model in part (b) to find a model for the rate of population growth.

(d) Use part (c) to estimate the rates of growth in 1920 and 1980 . Compare with your estimates in part (a).

(c) Fstimate the rate of growth in 1985 .