a bicyclist is riding at a speed of 20 mi/h when she starts down a long hill. the distance d she travels in feet can be modeled by the function d(t)=5t^2+20t, where t is the time in seconds.
a. The hill is 585 ft long. To the nearest second, how long will it take her to reach the bottom?
b. Suppose the hill were only half as long. to the nearest second, how long would it take the bicyclist to reach the bottom?

Question

anonymous · Answer

Put 585 for d

585=5t^2+20t

now solve for t

anonymous · Answer

Do i factor it?

anonymous · Answer

Yep make it look like this then factor it

5t^2+20t-585=0

anonymous · Answer

5(t^2+4t-117)

anonymous · Answer

ohhhhh okay. thank you so much!

anonymous · Answer

how about b?

anonymous · Answer

We haven't finished the first one yet =p

anonymous · Answer

9 seconds

anonymous · Answer

ill go with that

now, hill is half as long so just divide 585/2 and use that number in place of 585

anonymous · Answer

5t^2+20t=292.5?

anonymous · Answer

yep

anonymous · Answer

okay i got the answer! Thanks (: