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# help i cant figure out at all how to do this math

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An object is projected directly upward from the ground. Its distance in feet from the ground in seconds s=96t−16t2.

(a) After how many seconds will the object be 128 feet from the ground? (Hint: Look for a common factor before solving the equation.)

(b) When does the object strike the ground?

A. The object will be 128 feet from the ground only after ____ second(s).

B. The object will be 128 feet from the ground after ____ second(s) and after ___ second(s)

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s = 96t−16t^2

For the first part...  let s  = 128  ......so we have.....

128  = 96t - 16t^2     divide through by 16

8 = 6t - t^2      rearrange as

t^2 - 6t + 8  =  0      factor

(t - 4) )( t - 2)  = 0

So

t - 4   = 0                            or            t  -  2  =  0

t = 4                                                    t  = 2

So.....it will be 128 ft   above the ground after 2 seconds and after 4 seconds ...it is falling at the 4 second mark

For  part (b),  when it hits the ground,  s =  0

So....see if you  can solve this

0  = 96t - 16t^2

Let me give you a hint......if it takes 2 seconds to reach a height of 128 ft....it will also take  2 more  seconds to  reach the ground after reaching the 128 ft  mark for the second time

CPhill  Oct 21, 2017
edited by CPhill  Oct 21, 2017

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