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The equation  $y = -6t^2 + 51t$ describes the height (in feet) of a projectile launched from the surface of Mars at 51 feet per second. In how many seconds will the projectile first reach 108 feet in height?

 Apr 29, 2019
 #1
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OK....just set the function =   108    and we have that

 

108  = -6t^2 + 51t        rearrange as

 

6t^2 - 51 t   + 108  =   0         divide through by 3

 

2t^2 - 17 t +  36  =  0        factor....if we can

 

(2t  - 9 ) (t - 4)  = 0

 

Set both factors to 0  and solve for t   and we get two times

 

t = 9/2 sec    and  t =  4 sec

 

So  9/2 sec is correct....

 

 

cool cool cool

 Apr 29, 2019

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