SOLUTION: A bicyclist is riding at a speed of 18mi/h when she starts down a long hill. The distance d she travels in feet can be modeled by d(t) = 4t^2 + 18t, where t is the time in seconds.

Algebra ->  Quadratic Equations and Parabolas  -> Quadratic Equations Lessons -> SOLUTION: A bicyclist is riding at a speed of 18mi/h when she starts down a long hill. The distance d she travels in feet can be modeled by d(t) = 4t^2 + 18t, where t is the time in seconds.      Log On


   

Question 672615: A bicyclist is riding at a speed of 18mi/h when she starts down a long hill. The distance d she travels in feet can be modeled by d(t) = 4t^2 + 18t, where t is the time in seconds. How long will it take her to reach the bottom of a 400-foot-long hill?
We have to use the quadratic formula to find t.
I attempted the problem and i got 12.5 seconds.. but im not sure if im right
Heres my work:
d(t)=4t^2+18t=400
d(t)=4t^2+18t=400/4
t^2+4.5t=100
t^2+4.5t-100=0
(x-12.5)(x+8)
12.5 & -8 seconds.
is this correct?
Thanks(:
Answer by jim_thompson5910(35256) About Me  (Show Source):
You can put this solution on YOUR website!
You should have 8 or -12.5 seconds.

Because a negative amount of time doesn't make sense, you toss out the negative solution to get t = 8 seconds.