The number of U.S. cell phone subscribers (in millions)tyears after 1991 is approximated by

N(t) = 0.67t2+ 1.04t+ 2.5 (0 t 20)

1. Show thatNis an increasing function ofton the interval (0, 20).
2. What does your result tell you about the number of phone subscribers during the period under consideration?
3. What are the relative extremas?