Question

A projectile is fired straight up from ground level. After t seconds, its height above the ground is s feet, where s= - 16t² + 128t
For what time period is the projectile at least 192 feet above the ground?
Select the correct choice below and fill in the answer boxes to complete your choice.
OA. For the time period between (and inclusive of)
(Simplify your answers.)
OB. For the time period between (and not inclusive of)
(Simplify your answers.)
seconds and
seconds and
seconds the projectile will be at least 192 ft above the ground.
seconds the projectile will be at least 192 ft above the ground.

Answer 1

For the time period of  the projectile at least 192 feet above the ground is between 2 second and 6 second

Since s=128t-16t2, we want to know the 2 times where 128t-16t2=192, as those two values will be the begin and end times of the interval in question. We can rewrite that equation in standard quadratic equation form:

16t2-128t+192=0 or, simplified, 8t2-64t+96=0

or t2-8t+12=0

since it is now in quadratic form, we can solve using the quadratic formula:

t = 2 and 6

So the time period from approximately 2 second and 6 second has the projectile above 192 feet.

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