SOLUTION: the height of an object projected upward from ground level is given by h+-16t^2+128t where h is in feet and t is time in seconds. when will the object be 240 feet above ground leve

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Question 236860: the height of an object projected upward from ground level is given by h+-16t^2+128t where h is in feet and t is time in seconds. when will the object be 240 feet above ground level ...

please help i dont understand
Answer by Earlsdon(6294) About Me  (Show Source):
You can put this solution on YOUR website!
Starting with the given function:
h%28t%29+=+-16t%5E2%2B128t you are being asked..."at what time, t, will h(t) = 240 ft."
So you set h(t) = 240 and solve for t.
240+=+-16t%5E2%2B128t Subtract 240 from both sides and rearrange a bit.
-16t%5E2%2B128t-240+=+0 Now you have a quadratic equation that can be solved by various methods.
I would factor -16 first just to simplify the calculations a bit.
-16%28t%5E2-8t%2B15%29+=+0 Notice the change of signs when we factored the -16!
Now, from the zero product rule, you get:
t%5E2-8t%2B15+=+0 This can be solved by factoring.
%28t-3%29%28t-5%29+=+0 Apply the zero product rule again.
t-3+=+0 or t-5+=+0 so that...
t+=+3 or t+=+5
Why are there two answers?
On the way up, the object will reach 240 feet in 3 seconds (t = 3).
On the way down, the object will again pass the 240-foot level in 5 seconds (t = 5).
Does this all make sense to you?