SOLUTION: An arrow is launched upward. After t seconds,it's height(h) id given by the formula h=-16t+208t. After how many seconds will its height be 640 feet? Should this be solved using th

Algebra ->  Polynomials-and-rational-expressions -> SOLUTION: An arrow is launched upward. After t seconds,it's height(h) id given by the formula h=-16t+208t. After how many seconds will its height be 640 feet? Should this be solved using th      Log On


   

Question 34690This question is from textbook intermediate algebra
: An arrow is launched upward. After t seconds,it's height(h) id given by the formula h=-16t+208t. After how many seconds will its height be 640 feet?
Should this be solved using the zero-factor theorem? How should I set it up?
 This question is from textbook intermediate algebra

Answer by longjonsilver(2297) About Me  (Show Source):
You can put this solution on YOUR website!
i think you meant to have: +h=-16t%5E2%2B208t+?

If so, then when height is 640, we get:
+640=-16t%5E2%2B208t+
+0=-16t%5E2%2B208t-640+
+16t%5E2+-+208t%2B640+=+0+
+t%5E2+-+13t+%2B+40+=+0+
(t-8)(t-5) = 0
so t-8=0 or t-5=0
so t=5 or t=8

Two answers since the arrow reaches 640feet after 5 seconds, going up, then a further 3 seconds (the 8 seconds) it is again at 640 feet, coming down.

jon.