For the period 1990-2002, the number s (in thousands)lf cellular telephone subscribers in the United States can be modified by S=858t^2 + 1412t + 4982 where t is the number of years since 1990. In what year did the number of subscribers reach 50 million?

Question

For the period 1990-2002, the number s (in thousands)lf cellular telephone subscribers in the United States can be modified by S=858t^2 + 1412t + 4982 where t is the number of years since 1990. In what year did the number of subscribers reach 50 million?
  The choices are the year 1991, 1992, 1996, or 2000.
    I've tried the quadratic formula and tried finding the discriminant of the quadratic equation but I can't get one of the given answers. Can someone show me how to do this problem?

Reiny · Accepted Answer

The discriminant does not give you the solution, it is part of the formula that gets you the solution.

858t^2 + 1412t + 4982 = 50000
858t^2 + 1412t - 45018 = 0

t = (-1412 ± 12509.81695)/1716
= 6.46 years since 1990
= 6 years rounded to nearest year

so it would be the year 1996

Krystal · Answer

How do you get 50000?

Reiny · Answer

numbers used are expressed in thousands, you said so. So 50 million in thousands = 50 000 000/1000 = 50000